I agree witht he ip thing, but hes probally on a dynamic ip which will result in someone else being banned when his dhcp lease expires. If you block on higher level say that subnet your running into the same problem but on a larger scale. Ban the username and email address(es) maybe add a feature where a user has to post at least 10 times before they are allowed to make a private message(very easy, if you need help let me know). Also why not use a SQL statement to kill all private messages from the two users, this way anyone who has had the please of not opening it while at their job wont have to see it and our children will be safe.
Other than that, thanks for the directions on how to block the users, good luck admins!