So,
so far the suggestions for preventing spam bots – as verifying emails are easy for them;
Google's reCAPTCHA - probably the most user friendly, as users won't need to leave the page - ReCAPTCHA
IP Filtering - May cause issues with users using proxies (CPY's suggestion
Classic captcha - i.e. solving math / typing out a word - Securimage PHP Captcha
FunCaptcha - basically a puzzle captcha - FunCaptcha
I also found an interesting answer for someone trying to make a anti-bot from Javascript. > Here
The answer delves into his favourite type of anti-bot mechanism, a hidden check box. A checkbox for i.e. "I agree to Terms and Conditions" is hidden to the user, but a bot would detect this and automatically check it, thus revealing itself that it's a bot.
If anyone has anymore, feel free to reply to this post to add more!