You could do what I did and set a simple anti-bot question. I put a custom profile field that was required to sign up, but was not visible on your profile page. I then presented a list of 5 numbers (I did 1 thru 5, but any 5 numbers will work) and asked what the fourth number was, while setting the default answer to be the second number. I didn't get a single spam bot sign up after that.
I also did a drop down question that asked "are you a spam bot" and set the default answer to yes. It's amazing how these bots can break captchas, but can't answer simple questions.