I see Michael Hampton has released the latest version of his Bad Behaviour plug-in for WordPress. Bad Behaviour is a plugin for:
blocking malicious activity, including blog/wiki spam, e-mail address harvesting, automated cracking attempts, and more. It does all of this looking only at the HTTP request headers; for POST data, the content of the spam is not analyzed at all
I have been using Referrer Karma for this previously and it has proven quite useful. I’m going to try Bad Behaviour for a time now and see which of the two I prefer. One point in Bad Behaviour’s favour is that it has an admin page accessible from within WordPress. With Referrer Karma you need to do your administration seperately.