Quite often they degenerate into meaningless collections of junk visited by no one except automated spamming programs. I always find it sad when I visit a board in this condition. I mean someone put some effort into creating a community on the web, then for whatever reason neglected or abandoned it. The truly sad boards are those that were obviously active, useful areas full of vibrant communications which have degenerated into uselessness. I get every time I visit the long abandoned Marineland in Southern California.
Some boards require user registration .
In very strictly moderated boards, a moderator must approve each person who registers to access the board. This allows some measure of control over who can post. Security levels can further restrict what visitors can do. Good judgment in allowing people to join the group can obviate the need for extreme policing of postings. In other words, don't allow the bad apples into the barrel in the first place.