http://computer.howstuffworks.com/troll.htm

"he first step to dealing with trolls is learning how to recognize them. The following traits are clues:

* Does the person ask the same questions worded in different ways? Does the person ignore suggestions or responses from other members of the community? If the community has a frequently asked question (FAQ) section, does the person seemingly refuse to read it?
* Has the person posted inflammatory remarks that have no real substance to them?
* Does he or she make it a habit to post messages that include insults and vulgar language?
* Does he or she respond to other members in a purely negative, critical way?
* Does the person post messages that are generally off-topic? Does he or she seem to want only attention rather than discuss the topic at hand?
* Does the person resurrect old conversations or discussions that were once controversial within the community? Some trolls enjoy bringing back old arguments to encourage dissent within a group.
* When confronted with a counter argument, does the person in question change tactics rather than answer the points made by another member? Does the person employ logical fallacies within their posts?

If the answer to these questions is yes, there's a good chance you're dealing with a troll."