Weighting has been used for many years for determining the validity of emails - so why not apply it to signups?
Such a system would work, just like SPAM filters, on a points basis.
As an example, I find that those who signup up overnight are usually spammers from Asia. So I might give them -2 points for signing up between 10PM Western time and 7am Eastern. Certain email domains are almost definitely NOT spammers (comcast.net, verizon.net, etc.), so these registrations could be weighted to sail through if so desired. There are lots of tests which could be applied.