This hack will search for users who are in-active, have 0 posts, and/or do not visit very much, and delete them. The Admin can define the number of days before deletion happens and disable/enable each type.

Evo Notes: This is a semi-dangerous hack in the fact deletion of items cannot be un-done. Please, please, please back up your phpBB database before installing and running this. You never know what could go wrong, so just be safe
until you know it's working for you.