Trolls.
(Taken from Wikipedia)
In
Internet terminology, a
troll is someone who intentionally posts derogatory or otherwise inflammatory messages about sensitive topics in an established online community such as an
online discussion forum to bait users into responding. They may also plant images and data on networks that others may find disturbing (usually indirectly relating to the individual in person) in order to cause confrontation.
How to deal with trolls.
I think this picture sums it up perfectly:
If you spot a user that you think might be a troll, don't respond to them, don't attack them; just PM either support or myself with the name of the user and a link to the threads where they are misbehaving. Then we can do something about them.