I think a number of members post just to piss people off.
And as long as that behavior is tolerated, we'll continue to get it. What I have noticed over the last few years is that forums are becoming less tolerant of it. Photo.net is the one that comes to mind. Did a "180" compared with 5 years ago. So, it is possible to put an end to it. A lot of members will leave, and others will get banned.
So: the decision to be made on the part of the admin and moderators, is the level of tolerance. "Freedom of speech" being protected is nonsense. If some of the members acted this way in a bar, they'd be thrown out by the bouncers.