The question is: Is it possible disable only the forward stairs without lose the bus and the sounds of boarding/deboarding?
First, I suggest to download the latest version, because 1.8 is quite old, is not supported anymore and the 1.9 was a very large upgrade, with de-icers, etc.
Yes, you can do what you are asking, by editing the main passenger door to be at 0 height, but keep it otherwise enabled with the proper variables so, it's best if you start from scratch, use the "Reset" configuration option in the airplane configuration editor, and JUST set the main door height to be 0, so no vehicle will service it.