"Tony" tony@ttiger.co.uk
[...] The majority are from bots and most now can crack a Captcha. What has proved to be most effective is having a simple questions and answer, I just have four that randomly rotate on each sign up. Haven't had a single spam sign up since I done it.
When they crack that, http://textcaptcha.com/ is probably the next step.
Trouble is, any sufficiently popular site will also attract human spammers, so you also need some anti-spam measures, like not letting people post links just because they've signed up (like: no homepage link in the profile until after they've made N links).
As well as most of them being fairly crackable automatically, image and sound captchas also block users with even fairly minor disabilities, so I think they probably make one fall foul of the Equality Act 2010, but I also think we're still waiting for case law.
Good luck!