One solution, IIRC you get to press ESC for other reasons while using Excel, would be to fill screen (drag window to full screen size), and not use that last piece of the full screen.
I realize, not a perfect solution, but would eliminate accidentally leaving full screen.
You could also turn on the setting to Automatically Hide and Show the Menu Bar, in System Preferences/General pane, and also hide the Dock. The combination would give you the full screen space, and the accidental ESC would no longer be an issue.