The Disable staircases button will only disable the Rear Staircases on a parking spot with a Jetway. Assuming you are editing a spot on an apron ( otherwise disabling the Bus won't make much sense ), you can't disable Staircases, otherwise passenger wouldn't be able to exit/enter the plane.
Now, if the reason for disabling all stairs, was to use the 737 Embedded stair, since GSX is supposed to create vehicles even before you start a service, it will always create stairs, because it cannot possibly know in advance if you are going to use the Embedded staircase. However, if you put down the Embedded stair BEFORE you call a GSX service, it will use that one, and won't move the parking Staircase close to the plane.
