From time to time trolls pop out under their bridges and try to distract a discussion by posting inflammatory messages with the deliberate intent of provoking readers for their own amusement. Remember trolls feed of your negative emotions and enjoy your anger. DO NOT reply to them.
There's a nice video that explains this:
The Science of Internet Trolls: https://www.youtube.com/watch?v=6Zxy_dScjsM
Anonymity and trolls
The biggest problem is that this forum is anonymous, so a troll is sure that there are no risk nor consequences of his behavior. There's an old NY Times article that digs a bit into this topic:
Here's an excerpt:
Content providers, social networking platforms and community sites must also do their part by rethinking the systems they have in place for user commentary so as to discourage — or disallow — anonymity. [...] Some may argue that denying Internet users the ability to post anonymously is a breach of their privacy and freedom of expression. But until the age of the Internet, anonymity was a rare thing. When someone spoke in public, his audience would naturally be able to see who was talking. Others point out that there’s no way to truly rid the Internet of anonymity. After all, names and e-mail addresses can be faked. And in any case many commenters write things that are rude or inflammatory under their real names. But raising barriers to posting bad comments is still a smart first step. Well-designed commenting systems should also aim to highlight thoughtful and valuable opinions while letting trollish ones sink into oblivion.
History & Background
If you want to dive further into this topic I recommend the Wikipedia article about trolls which features a nice "Please do not feed the trolls" sign:
CollegeHumour produced a nice clip about trolls