I don't think its that greater problem. And there will always be those members who will try and slip under the radar, regardless of how many times you tell them. How many members who spam the forums actually stick around? Not very many. A lot of them come here for one reason, to gain sales, whether its against the rules or not. I don't think anything needs to be changed to be honest.
Maybe if it wasn't directly linked to the forums? A site which brings everything together (the forum, directory, and so on).