These days, bots don't do the spams. REAL HUMANS do the spam, acting as fake bots. As we all know, giving off simple questions enable them to research and answer them. But.. What if some questions are really hard that the humans can't answer? Logically, only the ones who want to actually sign up on the forums knows the question, but how?
Make use of the websites which contains IPs which blocks the spammers. For example, take. stopforumspam.com . You can either block the IPs you can find in SFS in your forum software, and another way is to give users a link to pass through a page which will give the activation code (same for all), which contains IP blocks (IPs of spammers etc from SFS). Thus, I'm calling it as a security check page. So, if a spammer's IP passes through the security check page, they're blocked from getting the code, since they are spammers. Normal users can pass through the check page with ease and get the code, activate the account and fap around to their own wish. Sometimes normal users are false-ly identified as spammers, so be cautious about what you're doing!
Or.. Download the files in SFS for usage in your phpBB forums, and let's see what happens. If the human spammers have been dramatically reduced, then you can cover up the mess by deleting the other spammers.
http://www.stopforumspam.com/contributions
Apparently there's a anti-spam plugin provided by SFS, please check it out!
Just my 2 cents.