There was a thread about it on the C74 forums who had the same problem.
Some interesting ideas in there. Perhaps it depends on how customisable the forum software is.
How about if a post was marked as spam by 3 users with 1000+ reputation it would be immediately trashed.
Moderating all ...
Actually, a good picture CAPTCHA is very solid. The only way a robot/program could interpret a good picture CAPTCHA is using character recognition. Now, computer vision simply isn't that advanced and likely never will be (the logic would be extremely complex).
The one used here might be too simple (not messed up enough) or it's not requested every single ...