Disabling Stairs disables only the REAR stair on a gate with a jetway. It can't be used to disable all stair on an apron, because otherwise passengers won't have a way to exit the airplane.
The correct way of doing this, is not disabling stairs at individual airports but, instead, configure the airplane to use an EMBEDDED staircase, setting some points that might work with small GA planes. This will automatically disable all staircase at all airports, because it's an airplane setting.
