Signing up for Defensio is almost as painless as, well, managing comment spam with Defensio.
You will momentarily be sent an email asking you to activate your Defensio account by clicking a link. Once you've done that, you will be sent a second email containing your unique Defensio API key, which you must use to configure the plug-in on your blog.
While you wait, why don’t you head over and download a plug-in for your blog now.