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.
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
__________________ maps.i40.com - Google maps sized for the 770 @ 150% - scroll below the map for features ukmaps.i40.com - Google UK maps - Pan and Zoom only. wiki.i40.com/Nokia770 - Random Nokia 770 notes.
Captcha : "completely automated public Turing test to tell computers and humans apart" (http://en.wikipedia.org/wiki/Captcha)