Yes, as explained on the updated manual, user customization is only available on airports that haven't been customized already. So no, right now it's not possible for you to turn off rear stairs at FSDT airports, but we could do it.
What's the reason for you wanting to turn off stairs on FSDT airports ? Just the rear stairs on gates with jetways ? Or stairs in general, because you fly airplanes with they own airstair ?
In the latter case, we'll add way to support airplanes with Airstairs, but that would be made as an airplane configuration option, it's not really related to the scenery.
The main reason for having a rear stair in the first place, even when there's a jetway was both because it's used for servicing, and because many airplanes don't have a rear passenger door animated separately from the main door, they open together and, seeing a passenger door opened in the void, was a bit unsettling...
