Hmm...interesting. I might consider to monitor this one silently (never roll around with a pig in the mud; you'll just get dirty and the pig will like it), and act with an attorney if his words become slander that you can prove have damaged your reputation and, as a result, your income.
Annoying people go away. Eventually.
Wow, man. To me it sounds like a guy on a power trip and in a bad mood. Myspace is a public domain, that being said here are two points to consider: 1) freedom of speech--if you ranted (which you claim you did not and I believe you but if anyone were to rant online...) the guy doesn't have a case bc seems like you know not to use a full name and if he doesn't like what was said, he can ignore it. Realistically, how many people are going to consult your myspace group before hiring the guy? No damage done to him--he's just trippin. 2) EVERYONE--Keep your personal and business social networking sites seperate--always. For anyone reading, if you request me on myspace or on LinkedIn I will say no. Not because I don't like you but professionalism and common sense tells me you do not tie your candidates, colleagues, and personal networking together. Imagine the scene in Ghost Busters where they cross the three laser beams and blow up a building.
If the dude keeps it up, just tell him he doesn't have a case, can't provide evidence of your supposed slander, and you'll contact a lawyer because now you have a case against him! If that does not work tie garlic around your neck and nail a cross over your bed. Sounds like this guy's a handful.