You would need to use notifications to detect if a sheet was renamed. If it was, then you would run some code that reverts the name change and perhaps tells the user that they cannot change the name of the sheet. Of course, this requires that you have the notification macro running already.
To find the right notification for a drawing you'll need to look in the swDrawingNotify_e enumeration and see if anything looks promising. If you wanted to be notified of an item being renamed in the feature manager tree, you could use swDrawingRenameItemNotify.
The Macro Library at our web site has some examples of notifications that might be helpful.