Stopping Disruptive Users in Online Communities?

Gabe the Programmer asks: "I'm the lead developer for a website and we have a community there for gay/bi/tran athletes to talk to each other and interact. Well, not surprisingly, because of the sexuality of our members and the site's high profile, we get a bunch of homophobic/racist/hateful trolls who come on to the forum for no other reason than to incite our members and waste their time. Most of the trouble is caused by a cabal of users who hang out on Fightsport.com, and over the past three years they've managed to drag down the atmosphere of our community substantially." If users are going to be rude and disruptive to your community, it might be worthwhile to ban them. Be forewarned, however! This may turn out to be easier said than done, since saavy users can always try and work their way around site bans. If you were a site administrator, how would you deal with intransigent users, and if you were forced to ban them from your site, how would you go about it? "It's gotten so bad that a lot of our longtime members have left the site altogether, and I personally dread visiting it many days. I know this is something of an age-old problem on the Internet, but what are the best methods to deal with this, both technologically and otherwise? When is it time to contact ISPs? Does that ever work? And what about the law? At what point is it appropriate to pursue legal action? I would really appreciate any advice from other Slashdot readers who are or have been in similar situations with online communities."