The admin private message hack allows admins to view, archive or delete any message by any user. On a phpbb board the installation time is 3 minutes. This hack adds a couple of options to phpbb admin panel.
You should add a clause to your user registration area saying you are using this hack, as well as alert current
users you have installed it. Otherwise, this may be
illegal according to the current ToS the user signed up
for and your country of hosting.
I have not installed it but the above makes it clear enough.
http://www.phpbbhacks.com/viewhack.php?id=200