It's not been a big problem for me, but the couple of times I've had a troll I just locked them into one forum only, and then gave them as good as they were giving (under an alias nick of course). They surrendered pretty quickly.
/me used to be an expert troll and can still be if I can pluck up enough anger and am awake enough to maintain it.
With my current setup there is only the dungeon that I could lock them into, but that isn't really what it is designed for, so I'd need to create a new forum to lock them in if I start having problems again.
I'm working for a big company in Sweden where trolling is a big problem sometimes. I've developed a small tool for our moderators, to check usernames against their ip. The tool does'nt warn on trolls but our moderators can easily scan for new nicknames in this database, to see if anyone has registered or posted with another nick before (posting is not required with this tool). The only problem is that this database grows fast, depending on the forums size and it's not completely foolproof. If some users uses the same proxy or ip, for example, there will be a lot more matches than we want.
- The good thing is that we've stopped some users before they even reached the threads... Most of the times, it really works...
I, for one, would be most interested in seeing that tool.