Administrators always have access to all matters in SB and they cannot be secured against them. To secure a file against a user, you have to remove their status as an administrator.

If you cannot remove the administrator status from the users in question, I would open a stub file in SB with basic matter information and only use it for billing. I would keep all data in separate email accounts, electronic files on PC, and detailed time records outside of SB. Then use SB for invoicing but do not include time details.