What I did (found it somewhere in phpbb's own community I think) was add a custom profile field with a required specific answer. In my case, it's a number, so only the correct answer is allowed. I even put the answer in text (ie seven when the answer is 7) within the field description, so if a person doesn't know the answer, it's right there. That immediately cut my spam registrations to almost nothing.
On top of that, I've always done account activation by admin only, so the few spammers that do slip through the question I usually catch on that. May not be reasonable for your board though if you get a lot of legit registrations.