I don't do anything with them. I figure, especially by the nature of my message board (and yours), many of them register, but only "lurk".
I track who visits the forums pretty closely, and I often see folks that have registered, but never posted, visit the forums on a regular basis. I'm hesitant to remove any I haven't seen, simply because they may be visiting when I'm not looking.
I also figure, aside from the space to hold the member data, there's little overhead from registered users who never post.
Lastly, sometimes I see such people finally work up the nerve to actually post, but it often takes them months before they feel comfortable enough to do so.
I wish there were some accurate way to guage whether people have actually returned to at least READ the forums, but given that they don't have to "sign in", coupled with AOL type IP addresses, there's just no good way to know.