i'm kinda new here, but it seams to me like some simple captcha under mesage input would do the trick. i know i know it's annoying but that's how it is...
i've been working on something like that. i don't really have time these days to improve it, but if anyone here have spare hour or two, you can make a skin that would match CF theme and implement one of those.
project site:
http://photocaptcha.us