Occasionally someone comes into the forum who doesn't really want to contribute to the forum in the same way that most people do. They tend to be very critical and opinionated and sometimes have the amazing ability to very quickly turn two people who normally get along fine in the forum to argue. Trolls can quickly turn what is normally a useful forum into a seething pit of anger. Unless you deal with them correctly.
The general way to prevent trolling is by positive reinforcement. When the Troll behaves in their trolling ways, ignore them. When they provide information that is useful, respond like you would normally. Remember that the internet and the people who use it generally do not deserve emotion. Especially not negative emotion. If you cannot handle a Troll, walk away. There's no point getting angry and upset as it will only carry over from the forum into your day-to-day life and drag you down.