Andrea, I did it but all in vain. I guess buddypress should talk with wp-recaptcha or akismat developers and come up with built-in akismat or captcha support for buddypress sign up pages.
And there’s no benefit of banning email domains or ip since every time they come up with new fake domains and ip address. There should be a permanent solution… “Spbloggers” is the biggest problem for those who use buddypress.