If they are in the moderation queue then you can use this:
http://xenforo.com/community/threads/how-to-mass-delete-users-awaiting-approval.35568/#post-404156
Otherwise there is no mass pruning feature for users.
You can put users into moderation for bulk processing with a query like this:
Code:
UPDATE xf_user
SET user_state = 'moderated'
WHERE user_id > 100;
In the above query it moves all users greater than user_id 100. That would setup all of those newest users for deletion in the moderation queue.