Thanks to Jay Rosen for linking to these sage observations from Anil Dash: If your website's full of assholes, it's your fault. Excerpt (but read the whole post):
How many times have you seen a website say "We're not responsible for the content of our comments."? I know that when you webmasters put that up on your sites, you're trying to address your legal obligation.
Well, let me tell you about your moral obligation: Hell yes, you are responsible. You absolutely are. When people are saying ruinously cruel things about each other, and you're the person who made it possible, it's 100% your fault.
If you aren't willing to be a grown-up about that, then that's okay, but you're not ready to have a web business. Businesses that run cruise ships have to buy life preservers. Companies that sell alcohol have to keep it away from kids. And people who make communities on the web have to moderate them.
And always bear in mind the difference between a jerk and an asshole: The asshole knows what he's doing.