One of the issues that internet forums have to deal with is trolls. This is largely the result of the fact that forums are anonymous; people will say things online that they would never say in person. Unfortunately a troll can ruin the forum for everybody else if they are determined so you are going to want to know how to deal with one in an effective manner.
A troll is somebody who goes to a forum in order to annoy or irritate the other people there. Usually they do this by making a post that they know will upset the people on that forum for example if it is a forum for fans of a specific sports team they may make comments supporting a rival team. There are lots of ways that a troll can irritate the people on the site. Their goal is to lure people into an argument. In most cases they are doing this for the attention although in some cases they may be internet marketers looking to sell something.
The best way to deal with a troll is to simply ignore them. The last thing that you want to do is to respond to their post and get drawn into an argument, this is exactly what the troll wants. If you respond they will just make more comments that are intended to annoy people. However if you just leave them alone and don't respond they will eventually get bored and move onto another forum and try to bother the people there instead.
The only problem with the ignoring the troll approach is that the not everybody is going to do it. There are some forums that have millions of members, at some point somebody is going to respond. This is why it is so hard to deal with trolls. Therefore the best thing to do is to not worry about it yourself. You won't be able to prevent others from responding to the troll and getting dragged into the argument but you can avoid reading the thread where the argument is occurring. This is really all that you are going to be able to since you won't be able to control everybody else.
One other option that you do have is to report the troll to the moderator. If enough people do this they will be banned and the problem will be solved, sort of. Unfortunately a determined troll can get around the ban by coming back with a different identity and start again. If he has been banned he will likely just be even more determined to irritate people and the problem will probably get worse. Which just brings us back to the original solution of ignoring the troll and not worrying about what other people do.