I have same problem. I have Q & A, and there a lot registration and spam every day... How is that possible?! How we can stop this? I think is some bug in phpbb3, they easy hack phpbb3, because, before I was using in Joomla Q & A -> and there was no SPAM. Now in phpbb3 - every minute...
P.S. You can find information here about this the problem.
The important thing is:
For instance, "Who do you see in the mirror?" is an effective question, while "What colour is the sky?" or "2+2 = ?" are not.