Captcha + E-mail Verify + Honeypot + Custom field validation (for example if you are asking for full name make sure it has at least two words in it and no weird characters) For us that keeps away 99.9% of bots. That does not keep offending or innapropiate content away. Mollom is a good option in the free tier, otherwise you can spend a morning setting up the above and keep your money.
Captcha + E-mail Verify + Honeypot + Custom field validation (for example if you are asking for full name make sure it has at least two words in it and no weird characters) For us that keeps away 99.9% of bots. That does not keep offending or innapropiate content away. Mollom is a good option in the free tier, otherwise you can spend a morning setting up the above and keep your money.