Is this user in cached mode?
If yes, then you can :-
a) Use Outlook Spy and delete the hidden message on the inbox, and do another mailbox synch.
b) Use EVPM and zap the mailbox
c) Delete the users OST file, and allow Outlook to create a whole new one the next time it starts (and you'll then get a new hidden message, with your policy settings in it from the server side version of the settings)
Working for cloudficient.com