If there some trolls build a group, they can put down every thread heavy. These people are only go fo cofrontation.
If he/she is actually trolling and constantly posting off-topic messages, then they would be breaking the forum rules and you could report them. Otherwise, if their opinions are valid and coherent with the threads that they are posting them in, you can not do anything. If you are starting your own thread(s) and want to avoid certain members, then make sure to make it self-moderated.