Require new members to pass a captcha test. This should solve most of the problem. BTW, instead of generating a new captcha for each visitor just make ~1k of your own "as random as you can get" captchas to save cpu time.

Captcha : "completely automated public Turing test to tell computers and humans apart" (http://en.wikipedia.org/wiki/Captcha)

If you do that, don't forget to add a workaround for site empared members. Although, I'm not sure the 770 it's self is currently site impared friendly, that may change over time.

-Jason