I've made an spambot detector for use for everyone, you can use this for any registration form or anything else, it checks the IP and E-Mail, and if an match has found in the stop forum spam database it gives true.
I've also included an domain blocker, so the main domains like mail.ru can be blocked standard.

You can find an example here: (the source is included)
http://guildwarsholland.nl/phphulp/testspambot.php