Not a bug, just a suggestion:
Ever since you switched to phpbb forum software there is an addon that allows to get rid of all attempts to sign up using spam forum email registrations called stop forum spam.
Although they don't actually manage to post on the forum they do sign up and take database space.
I am aware it barely takes space but it will make the forum sign in slower as time goes by and more such spammers sign up. I use this addon on my forums, very easy to install if you have 3.1.x version.