There is a plugin for some forum software that allows you to cause them all sorts of hell.
Sometimes it will error out, other times it will work, it gives the illusion your site is having lots of problems and depending on how you set it, drive them mad. Most will simply tire of it, especially if you ramp it up slowly. Some will figure out something is fishy though, at which point you have to do something more, but it sure is fun watching them squirm. I reserved this only for those who are there to causing trouble but are very aware of the thin line they walk and make sure to just stay on safe side of it, but only just.
Another option is a rep system.
While it can be abused, it is quite effective against most trolls. Unfortunately some decide that is fine too and work to get a low reputation, competition at it's finest. You need to set the ban level at a pretty easy level to cross to avoid this.
Bottom line, don't let them live on your boards, no matter how much their friends complain, or how much they contribute.