Why is a core group of instigators allowed?

I have noticed that there appears a core group of posters that do nothing but throw insults at other posters and cast off topic disruptions into various topics. From what I have seen, they add little to the forum and detract greatly from it.

Why is this allowed?

As a new person here, all it has done is discourage me from posting. That's all I have to say about that. Maybe someone "official" can post an answer? (I'm not for censorship or whatever, but there has to come a time when you have to ask yourself when is enough enough?)