IIRC, up until 0.85, the only way to get n/a was through input general and esc in input this game. Well, that's ignoring the ctrlr files; that was the other way.
But 0.36 did not have ctrlr files, so, you'd have to unmap in both "inputs general" and "inputs this game". But then you have remap in all the other games.
IMO, having n/a is mostly so later on you (or someone else) would know immediately that that button was not used, yes?