EDITED JAN 2009. (
Co-mods, please read this related topic.)
A couple of days ago, I got a PM asking how can a member un-register from the forums. Sorry, we're reverting to our former policy of generally not deleting user accounts. There are a number of technical and other reasons for this, one of which is there's always a chance the user will change their mind.
Exceptions would be accounts that are obviously spambots or those that we suspect to be automated registrations. We've already deleted thousands of such accounts.

That's why new members get a user ID # that's 2x (or more) the total # of registered members shown in the board stats.
To my PK co-mods, please read the Jan 9 2009 "warning" on my account for details. 
I do know that as recent as Jan 2008, PK mods and admins were getting repeated requests from some PK-ers to have their own accounts removed. I think I can see why the admins at that time acquiesced to some of those requests.
(This is not a criticism of the previous mod teams' efforts to combat robo-accounts. It only shows the growing sophistication of spam / auto registrations these days.)
Last month, I did delete a duplicate and recent account made by a PK oldbie, who PM'd me after she found out she could still access her old one.

That duplicate account hadn't made any posts.