Sometimes moderators will close a thread when it gets to be more of a problem than an asset for the majority. Most of the forums I have been on, have tried to keep things under control. If you get too far out of control, then the radicals will create an atmosphere that run off the average users. If the forum is too tightly moderated, then people will go elsewhere because they feel bound by the rules.
I have seen several places that lost their interest because they became nothing but porn and name calling. That is the tyranny of the net. What some fools would fear saying to your face, they have no problem saying on a forum. And being cruel and nasty is easy when you are a nation away. So many of the places hold strict rules of acceptability, with religion, politics, and insults being verbolten. Amazingly, one of my favorite places is one where I can routine talk politics, religion, and almost any other subject without fear of it getting out of hand. The moderators will allow civil discourse and the people there are mature enough to argue serious subjects of philosophy without getting personal. I do not think all of the people that come here are up to that standard.