One of the problems that drove me away from Wordpress to begin with was the number of spam user registrations. It was getting downright insane. To combat that, I thought installing WP-United, and integrating user accounts, might stem the flow if I installed anti-spam measures in phpBB. That didn't really happen. It just meant that I had to remove these spam accounts from two places now instead of just one, as, even though users should be registering through phpBB in general, they can still register via Wordpress as well. So I installed WanGuard to see if that helps.
Let me know in comments if you have any other solutions you've tried, or have any experience with WanGuard.