I don't allow anonymous users to post comments (I don't think I do, anyhow!) - you might consider something similar. Personally, I don't see any advantage in allowing those people to leave comments - you don't even know who they're from!

Oh wait, I think my site DOES allow comments from anonymous.... oh oh. I'll go check that out now.

Edit ---
There, I've disabled that shite on my site now. No anonymous posters!
/Edit ---

James Addison
http://www.pjsoft.ca
Developer Extraordinaire, and Gimp Cyclist.