On a related topic, we've had a few spam subscriptions in the last couple of days.
PHPBB gives you a very convenient way to delete the user and all their messages in a few clicks.
I changed the antispam question from one about Dai-X to something else that was easy and Star Fleet-related.
Maybe they went on the site, did a manual subscription and then added the working answer to some database or other for spambots to use.
Edit: Well, that seems to have worked.
Dream big and bold and daring.