A pertinent comment from Wikipedia about trolls.

http://en.wikipedia.org/wiki/Internet_troll


Trolls can be existing members of a community that rarely post and
often contribute no useful information to the thread, but instead
make
argumentative posts in an attempt to discredit another person,
concentrating almost exclusively on facts irrelevant to the point of
the conversation, with the intent of provoking a reaction from
others.
The key element under attack by a troll is the forum or group's
recognized and agreed upon power structure.