This turns out to be very easy: Edit > Keyboard Shortcuts > Disable all menu access keys. (from)
This has been bugging me for ages, I wish I'd looked earlier. It is useful because the bash shell supports lots of nice (emacs-derived) shortcuts for moving the cursor, such as Alt-F and Alt-B to move forward and backward by a word; since I'm used to these working, whenever I go back to gnome-terminal it confuses the nubbins out of me. I shall be confused no more!