In this Information Week article , Cory Doctorow, author and co-editor of Boing Boing takes a look at how to deal with trolls - people that are toxic to communities due to their aggressiveness and persistent poor behavior.
True to form, Cory uses a geeky radiological metaphor to explain the tactics...
Discussion groups are like uranium: a little pile gives off a nice, warm glow, but if the pile gets bigger, it hits critical mass and starts a deadly meltdown. There are only three ways to prevent this: Make the pile smaller again, spread the rods apart, or twiddle them to keep the heat convecting through them.
This reminds me of Clay Shirky's article "A Group Is It's Own Worst Enemy".
In the end, there are no easy answers. Cory reviews technical (ratings), non-technical (contacting the person) and a combination (disemvowelling and troll whispering - you have to read the article). What is clear is that communities need active management and people who can work with the group, using multiple means, before complete meltdown occurs.