Just some ideas:

Limit the number of forum posts to one or two per day, especially new topics, for a while, say a couple of weeks, after they sign up.

Automatically flag any posting by a new member that has a URL in it.

Automatically flag any posting by a new member that has certain key words in it, like "soccer" or "TV", especially in the title.

Limit new users from posting more than a couple of posts per day until they can successfully answer a number of randomly selected Questions of the Day.