You likely won't ever get it to say 0% used, some of that partition is used for temp storage.
To wipe the device completely needs an ALTBoot from a technician (I am not allowed to disclose how it is done on the forum, 2nd level support can tell you by phone if you are on contract)
To Wipe the mailboxes you can do so as admin on the device or via CWIS by choosing Files and Folders in the same area I showed in the above screenshot.
A Secure Disk Image Overwrite should clean most temporary files.
The only way something got into a mailbox after it was disabled would be that the machine had not as yet updated the template list, reboot or hit refresh template list on the Workflow scan screen. You could also in addition kill the Allow scanning to Default Public Folder by selecting Scan Policies (again, same area as above.)