There is an actual historical group, called the Hellfire Club. See:
Welcome to the group, Ed!
I encourage you to post in the Introductions section and let us know as much about yourself as you’re inclined to share. (I don’t remember if that’s the exact name of the section, but something like that.) You probably noticed that we have a “one topic per week” rule–the introductory post doesn’t count against that, so please don’t hesitate. I approved your post, and you have one more post to go (or maybe two) and then your posts will automatically go up when you submit them. That’s our low-tech “firewall” against spambots joining and, you know, spamming us. If your post doesn’t get approved right away, that’s my fault, or Jason Pratt’s. I’ll try to keep an eye out, but Jason’s really more diligent than I am.
I’m going to tag [tag]JasonPratt[/tag] and [tag]Paidion[/tag] for your question. I could attempt an answer, but they know a lot more about the early fathers than I do.
Again, good to have you and I hope you find everything you need in your quest. 
Eternal conscious torment as a Christian doctrine goes back at least as far as the 2nd century (the 100s), and although there’s some dispute as to whether some patristics held it or not (such as Irenaeus and Justin Martyr), others are quite indisputable, Tertullian being a big example. This is aside from the question of canonical meaning; also aside from extra-canonical texts that aren’t strictly patristic (such as apocryphal texts) but still were widely regarded as orthodox.
As a Jewish doctrine, ECT goes back into the 1st century and beyond, but there’s the usual spread of differences and some probable oversimplifications in writers (like Josephus) reporting positions.
As to why the writings of orthodox trinitarian (and proto-trinitarian) fathers didn’t carry the day for universal salvation, that’s partly because they weren’t trying to dogmatically settle that question; even when using universal salvation as a maximum Christological argument, they were aiming for the Christology and other related theological points. It’s also partly because many of them held the doctrine of reserve, where they deemed it better to give common people simpler ideas even if inaccurate ones, on the theory that uneducated and spiritually immature people would only abuse the truth – but then that affected the shape of the writings they left behind. Stronger threats were naturally more useful for opposing and inoculating against heretical positions (regardless of which side thought the other was heretical), and that played its part. Relatedly, after the days of Nyssa and Theodore of Mopsuestia, the western half of the Empire was under constant raiding threat by pagans (who were also sometimes neo-Arian Christians), and this played a part in people’s attitudes. As the catechetical schools denounced each other over subtle Christological matters, and flung the same denunciations at authorities trying to stake out a central orthodox position, a crisis developed over which institution should best be capable of passing along or (where necessary) rendering dogmatic judgments about what counts as correct and incorrect belief and about how true Christians should be identified as in common union together. This came down to the Roman bishop in the seat of Peter, and the Roman Emperor in Constantinople, both of whom had motivations to paint chief authorities in the catechetical schools (like Origen and TheodoreMop) as radical heretics to be excomm’d posthumously. Eventually this led to the split in Orthodox Catholicism between the Roman Catholics on one side and Eastern Orthodoxy on the other. And that split would necessarily encourage the idea of deadly final penalties for those outside the faith on either side.
There is also some evidence, although I haven’t looked into it myself yet, that after the fall of Constantinople in medieval times, Russian Orthodoxy led a renaissance that borrowed heavily from Roman Catholic ideas – but the ruthlessly cruel byzantine scheming of Byzantium (Constantinople) doesn’t give me much expectation that the leaders there managed to keep some pure doctrinal light only swallowed by Russian tampering afterward.
So the too-simple answer is that there is no simple answer. Sorry. 
That depends on what you mean by the question, and even then you’re going to get some different answers. The simplest and I think most historically correct answer is that “hell” is simply “hole” or “pit” in pre-English Germanic languages, and translates “sheol” pretty literally; consequently, it translates anything in the NT that also renames or translates “sheol” well enough. That includes Gehenna, which is connected in the NT to “hades” being used as an Greek alternative term to “sheol”. If anything, “hell” translates “Gehenna” too shortly; it ought to be the hell of lamentation, “Ge/ga” being one of the synonyms for “sheol” although usually used for a more open pit or indention, thus also for “valley”, thus also for the geographical feature of the valley of the (sons of) lamentation around 3/4 of Jerusalem.
Whatever those terms can mean Biblically then, “hell” can mean Biblically. There’s nothing at all wrong with the word. Gehenna became hell when Christians started teaching in ancient Germanic languages, however early that was.
If you mean, when did Gehenna start meaning a finally hopeless punishment, that was one of the several Jewish positions predating Christianity, using the physical valley of Gehenna as an analogy; and the question of when some Christians picked it up is just the question of what idea is original to Jesus’ teaching and how far the apostles kept to it before multiple ideas show up among different respected orthodox teachers (and why that happened). So, no simple answers there.
Jason, I have seen this claim more times than I can remember. I have never once seen in a document pre-dating A. D. 30, however, the Greek word “Gehenna” used to refer to post-mortem unpleasantness.
The earliest I have ever seen the Greek word “Gehenna” used to refer to post-mortem unpleasantness is in the mid to late 2nd century A. D.
I think Gehenna is the Lake of Fire about which John wrote in Revelation.
Jesus gave many warnings about the dangers of going to Gehenna:
Matthew 5:22 But I tell you that every one who is angry with his brother shall be liable to judgment; whoever insults his brother shall be liable to the council, and whoever says, ‘You fool!’ shall be liable to the Gehenna of fire.
Matthew 5:29 If your right eye causes you to sin, pluck it out and throw it away; it is better that you lose one of your members than that your whole body be thrown into Gehenna.
Matthew 5:30 And if your right hand causes you to sin, cut it off and throw it away; it is better that you lose one of your members than that your whole body go into Gehenna.
Matthew 10:28 And do not fear those who kill the body but cannot kill the soul; rather fear him who can destroy both soul and body in Gehenna.
Matthew 18:9 And if your eye causes you to sin, pluck it out and throw it away; it is better for you to enter life with one eye than with two eyes to be thrown into the Gehenna of fire.
Matthew 23:15 Woe to you, scribes and Pharisees, hypocrites! for you traverse sea and land to make a single proselyte, and when he becomes a proselyte, you make him twice as much a child of Gehenna as yourselves.
Matthew 23:33 You serpents, you brood of vipers, how are you to escape being sentenced to Gehenna?
Mark 9:43 And if your hand causes you to sin, cut it off; it is better for you to enter life maimed than with two hands to go to Gehenna, to the unquenchable fire.
Mark 9:45 And if your foot causes you to sin, cut it off; it is better for you to enter life lame than with two feet to be thrown into Gehenna.
Mark 9:47 And if your eye causes you to sin, pluck it out; it is better for you to enter the kingdom of God with one eye than with two eyes to be thrown into Gehenna,
Luke 12:5 But I will warn you whom to fear: fear him who, after he has killed, has power to cast into Gehenna; yes, I tell you, fear him!
In the parable of the sheep and the goats, Jesus says that King will say to the “goats”, ‘Depart from me, you cursed, into the lasting fire prepared for the devil and his angels." I think this “lasting fire” to which Jesus refers, is Gehenna or the Lake of Fire.
However, I think the purpose of Gehenna is that of correction. It is part of bringing the hardened and rebellious to repentance and submission to Christ . It is a very severe correction, and it would be better to avoid it if possible, and Jesus seems to have suggested ways to avoid it.
“Gehenna” is not the same as “Hades.” The latter means “the grave,” although there were people, even among the Jews, who thought it was the place to which people went after death, and Jesus employed that belief in his parable of the rich man and Lazarus, the point of the parable being that even if it were possible for someone to return from the dead and tell the living about the suffering of some people in Hades, they would not repent.