With 2010 there is the get-mailboxfolderpermssion to audit then remove-mailboxfolderpermission to remove. None exists for 2007, the only way to modify mailbox level perms programatically is via EWS.
The only other idea that I can think of is to create a generic mailbox after ther user is termed and them export the current users mailbox into a new mailbox.
James Chong MCITP | EA | EMA; MCSE | M+, S+ Security+, Project+, ITIL msexchangetips.blogspot.com