Well one of the ways I decided to keep them away was via a small bit of javascript within the registration page that would trip up certain types of spam bots but not affect normal users at all.
Then I also came across a type of captcha that I had never seen before called RoCaptcha, yes RoCaptcha that isn't a typo
It is where the user has to rotate the image displayed to an upright position! It is fast and easy for your average user
http://rocaptcha.com if you wanna check it out youself







