If you want to run a forum, then make it private only so you don't have to worry about spammers, etc. You invite whoever you want from other forums.
In such a case forum will never be a real forum, it's a discussion that can be coreesponded by e-mail
Actually, there should be some time taken to moderate every forum, and of course, it requires some work on your part, or on someone you trust, or hire to do such a work...
