Have you ever did a purge on your forum? As in go through old topics/posts and delete the ones that are not needed or pointless? I remember one of my first forums was spammed to hell and back. There was thousands of topics and posts made by groups of people who just wanted to make a problem. So I purged the forum and deleted all of their accounts. I also banned them from ever creating a new account. Ever since I established rules to where users have to make a certain amount of posts to be able to post topics. And I think for one forum I set a rule for new users that they can't go past 10 new topics and 20 new replies a day until they meet a certain point of posts and activity where I see they're safe.