That is actually quite an interesting issue. Does the forum "belong" to it's creator, or to the members? Is it like inviting a group of people to your house - in which case they would be rude to complain about the music you play or the curtains you've chosen - or is it more like a "clubhouse" that nobody owns but is a communal space?
I suppose this would depend on the context. For example - I've participated in some forums which are hosted by a specific author, and is sort of about them and their personality. In that case, I can understand that it would be a case of "if you cant stand the heat, get out of the kitchen." But in this case, it sounds like a pity that the community is at the mercy of a rather immature individual!
Since the forurm "owner" is paying for the web hosting, possibly paying for the software, and put in the time to set things up and promote it, I think it's fair to say he owns it. He therefore has the right
to run it pretty much as he wants, within whatever legal restrictions there may be on harassment and similar issues. But what one has "the right to do" and what is "right to do" are not the same things.
In any case, if a group of you are not happy with how a forum is being run, it is not all that difficult nor expensive (you can get decent shared hosting accounts now for $5/month or so* and a domain name is something like $10/year) to host your own forum and migrate there with like-minded members of the other forum. Of course, then you may find you have to deal with the time/effort to get it known and attract enough members to provide enough traffic to make it worthwhile.
* and you can host multiple forums/blogs on that one account