With all this talk of the pink canned stuff (and a quick peek at the shoutbox) you could implement an e-mail filter option to the Registration Queue / Banned E-mails areas.
It was mentioned in the squawkbox about not banning @gmail.com but if you had a filter you could divert anyone signing up with a @gmail.com to the Registration Queue (if by default you don't have the RQ enabled).
Probably needs a little more thought but I thought I'd jot this down while it was fresh.
