BotPoison is a simple tarpit that drastically slows down bad Web crawlers (e.g. spambots). It works by creating tens of thousands of slightly different links to itself, to quickly dominate a crawler's queue. Well-behaved bots (e.g. search engine crawlers) are NOT affected, as it uses the
<meta name=robots> tag to make proper bots ignore it.
Note that bots could easily be programmed to detect BotPoison; however, it would be just as simple to modify it to keep up.
If you would like to make your site spambot-resistant, you can simply insert the following HTML anywhere in the site:
<a href=/path-to-botpoison.php style=display:none rel=nofollow>link</a>
Download BotPoison and host it on your own site, then change the link.