Unfortunately not, you would need to know the usergroup or email address or username, it's fair to say that you could use vbulletins built in prune users to prune say all accounts older than x days with zero posts.
I'll see what i can find on the UPDATE USER query
--------------- Added [DATE]1319340413[/DATE] at [TIME]1319340413[/TIME] ---------------
Did you use Impex to import your database to vbulletin?, if not and you still have the database it might be wise to go back to the backup you made before adding all your new data and then use impex to import them.