Replacing the forum software for a spam problem is sort of extreme, don't you think? That's kind of like replacing your car because it needs new brakes shoes.
There are add-ons that will add a CAPTCHA code box for signing up to the board in the first place. That will stop 100% of spam bots right there, preventing them from signing up in the first place. Not much can be done about stopping the bots that were set loose after an actual person set up an account, as it happens sometimes. But those are easy to deal with.