If you want my opion, add something to your AUP that says the website hosting must be used for a website, and can not be for file storage. After that, Termination his account for Violation of the Aceeptable Use Policy and watch out for duplicat accounts like he has. If you share the script with me like wher he got it or etc, i could look into it all for you. But if you want proof, see who is accessing his website through the log's Ie: firewall and etc if you want hard information ebfore terminating him.
Jeff