This Lifehacker article goes into how best to deal with spammers, scammers, jerks on Facebook and Twitter, gaming cheaters, malware purveyors, undelightful commenters, fake product reviewers, haters, and arguers.

But the best advice is on dealing with trolls:

The absolute best way to avoid the leagues of morons on the internet — particularly the troll variety — is to just ignore them. They aren't attacking you, they're attacking boredom, and they're hardly worth the energy of even engaging, particularly because that's exactly what they want. If you get locked in, you can weasel your way out with dignity, but the more you realize they aren't worth the effort, the happier you'll be.