X****r bot is your main enemy. Among other things, he can:
- find the forum on your site,
- select the best forum category to post to,
- decode many types of CAPTCHA,
- complete email verification,
- create accounts on free mail services.
Fortunately, X****r is quite expensive and his site isn't translated to English yet.
Recommendation: if you write your own protection, expect that spam bots are smart and humans help them.
Next: Textual Confirmation
Home: phpBB Antispam HOWTO