I need clear Mailbox from a User, it's possible?
I'm not sure what you're asking... there are only 3 ways to delete messages.
1. Login as the user into PCA and delete the messages
2. Login as the user using an IMAP client and delete the messages
3. Login via the phone as the user and delete the messages
There isn't a way to do it administratively for multiple users with an admin account, if that's what you're asking.
Hope that helps,