PHP My Spam Fighter
It makes/fills email extractors/spammers' programs with thousands of fake email addresses endlessly dynamically generated by phpMySpamFighter. So even if your site visitors post their email addresses in plain format, spammers will give up searching for correct ones. It may cause Denial-Of-Service attack back to their programs.In fact, it fights not only spammers but also your attackers who use the similar tools to probe your web sites.
Pretty Easy! Just insert the following html code in your web pages:
<a style="display:none" href="phpMySpamFighter.php"></a>
Making display:none makes Search Engines ignore the link. Only spammers's softwares follow this link and will get sucked. If you're good at PHP, tweak the code to maximum security!