Hi all. I've made a version of bad behaviour to b2evolution. It should work with 2.4.x and 3.x series.
You can download it at: https://sourceforge.net/project/showfiles.php?group_id=160495&package_id=325820
Bad Behaviour is a tool to prevent referer (and a lot of types) of spam.
"Bad Behavior is a unique way of blocking link spam and the robots which deliver it to your blog, forum, wiki or CMS. It operates in a completely different manner from other spam prevention services and is meant to operate alongside and in conjunction with other spam prevention software."
More at: http://www.bad-behavior.ioerror.us/documentation/
Let me know if it works for you! The plugins provides a widget too, to show to the world how many spams bad behaviour blocked. Look at http://devlog.waltercruz.com/ for a sample (I'm using this plugin for some weeks).