If a user has full access to a mailbox that means exactly that - full access. You cannot restrict them in any way.
However unless they opening the mailbox as their primary mailbox they will not be able
Veeam is happy to provide a free NFR license (for 1 year, up to 10 users). This license allows for the non‑production use of Veeam Backup for Microsoft Office 365 in your home lab without any feature limitations.
A list of top three free exchange EDB viewers that helps the user to extract a mailbox from an unmounted .edb file and get a clear preview of all emails & other items with just a single click on mailboxes.
This video shows how to quickly and easily add an email signature for all users on Exchange 2016. The resulting signature is applied on a server level by Exchange Online.
The email signature template has been downloaded from: