When my forum started to get hit by automated spam bots I quickly made a small modification to the sign-up page as you need to be registered to post.
Now when bots try to register they always fail and it gets logged to my apache's error.log, normal users are not having any problems.
It is a simple extra form where it asks a simple math sum, and you need to enter the correct answer. (I based it on the "Did you Pass Math" mod for Wordpress, but I used server-side openssl for challenge/response)
You can have a look at: http://www.savvy.nl/blog/forum/register.php and check if you can write an automated script to register an account.
Actually it's not that hard if you evaluate the form question.. but hey it works, i got ZERO spam 
If anyone is interested in the mod, PM/email me. I'm not making this public as it will increase the chance that bot coders will pick it up and write something for it.