When {kata} is used by itself, it just means “down”.
When {kata} is used with a genitive object of a preposition, which normally by itself would mean “of the noun”, the preposition indicates down-something-the-noun. Down-what differs according to the context. Literally it would be ‘down-from-the-noun’, but there are a number of common metaphorical applications, too: it can indicate an oath, as though someone is laying their hand down on something to swear truth; or it can indicate hostile opposition, like when the gates of hades will not stand down-upon or down-strong against the Church.
When {kata} is the preposition of an accusative prepositional phrase, like in the oldest known Gospel titles (which simply read things like {kata} Luke for example), it shifts to a different metaphorical application of ‘down from’: the text was passed or written down from Luke, in other words the text is written by Luke. By extrapolation the term can also mean “in accord with” or “in agreement with”, which occasionally has some interesting applications in soteriology! In English we commonly combine the concepts to translate the Gospel titles (for example) “According to Luke” or “According to Mark”: Mark and Luke are passing their versions of Jesus’ story down to us (metaphorically speaking), and the text agrees with what they intended to say while doing so. (In current English urban slang, the object of the verb is “down with” the subject! 
Yo, are you down with Jesus and His gospel, blood?)
(It’s also a super-common prefix, developed from abbreviating an originally separate preposition {kata} together with its object into a compound word, which sometimes leads to some curious further metaphorical meanings.)
In this case, the textual transmission consistently reads the prepositional phrase {kata tôn toioutôn}. (There are some transmission variations in the first part of verse 23, in how the list of fruits of the Spirit end, but not in what we call the second half of the verse.) That’s {kata} with a genitive plural direct article plus a genitive plural noun serving as an adjective.
That adjective is a little weird and hard to translate into English. It’s a compound word formed by a dative plural direct article {toi} and {houtos}, which is itself a plural formed from the nominative direct article {ho} and {autos}
So the whole phrase literally reads {kata} of-things of-pertaining-to-these-these-things! Except the "of"s don’t actually count anymore because of the {kata}!
And now… you are insane. 
http://www.wargamer.com/forums/smiley/dizzy2.gif Sorry.
How does this resolve back out into a sensible meaning?
Ignoring the genitive form for the moment, {tôn toioutôn} is a Greek way of emphatically stacking up self-referential “things” for a meaning like “things such as these things” or more shortly “such things”, but with a lot stronger emphasis: think of it as “most certainly such things”.
{kata tôn} is the actual prepositional phrase (with {toioutôn} describing {tôn}), and since “things” (and its adjective) are in genitive form, that means {kata} either means literally down from, or metaphorically means swearing an oath emphatically, or metaphorically means strongly against (like pressing or looming down over or down upon).
{ouk estin nomos} simply means “a law is not”, with the negative “is not” being fronted for emphasis, and the “not” being fronted to show it goes with the verb (instead of “is not a law”). So it’s backward compared to our grammar but there are reasons.
After that, it’s just a matter of testing which optional meanings make the most sense.
1.) “Physically down from emphatically such things, most certainly is not a law (or the Law).”
2.) “By emphatically such things (I swear), most certainly is not a/the law.”
3.) “Against emphatically such things, most certainly is not a/the law.”
Option 3 makes the most sense to most translators, myself included. Paul isn’t swearing by those fruits of the spirit that law (or the Torah) doesn’t exist; and he isn’t trying to say that those things certainly didn’t physically send down a law (or the Torah).
He’s trying to say that there is certainly no law (not even the Torah) against such things the Holy Spirit cultivates, as self-control, patience, faithfulness, peace, etc. Whereas people who keep doing the things in the previous list (vv.19-21) aren’t going to inherit God’s kingdom. The Law is against such evil things, which do not come from the Holy Spirit, but the Spirit leads people to habitually do other things which the Law most certainly is not against.
(Paul probably isn’t saying here that the Law doesn’t cultivate good fruits, the Spirit does; but I expect he’d agree with that, especially in Galatians. )
But the evidence doesn’t support some kind of conspiracy to omit adultery, to implicitly suggest that was permissible.)