Disabling rear hatch button on fob?

Other than gluing it or putting tape over it I don’t know. Maybe some electronic techies here might know how to open up the fob and sever its action.
You do know you can adjust the opening height of the hatch?
I have mine set for the very same reason you mentioned it stops about one inch away from the hanging hardware in my garage.

OK, this! I've had this happen twice now, and it's super unnerving when you return to your car in a parking lot and the rear hatch is wide open. You think it's related to an accidental button press of the fob?

That’s what mine was doing, I pulled the button out of the fob. If it opens now, I’ll know it’s a ghost

Maybe try a silicone case from Amazon. While it isnt a 100% fix, it does make the buttons less easy to press. Gives protection for the key too.