Is there a way to disable the sorting on click within mail.app?
Someone might know if you can disable the sorting, but if that isn't possible, you can create a keyboard shortcut to switch the sorting back. 

Would that be something the user could type?


Go to System Preferences / Keyboard / Keyboard Shortcuts /
Applications.
Click the + button.
Chose Mail.app
Type Date into the Menu Title field
Select an easy to press and remember combination. (You can't overwrite already existing shortcuts. When I tried this to    test if it would work I used command shift    D )
Click "Add" and you are set.