I would suggest script steps in an "Open" script:
- Allow formatting bar [Off]
- Show/Hide Toolbars [Lock; Hide]
I'm not sure whether you'd need both or just the second one, though.
That worked I did need both
You shouldn't need the first script step unless you have manually selected the Aa button to reveal the formatting tool bar. Deliver the file with the formatting tool bar deselected and Toolbars hidden/locked and there should be no way for the user to get to the Aa button to show this additional tool bar.