It's not possible to disable the Bus globally, and it would be wrong if it could because, in order to replace a bus, there must be a custom WALK/IN path for passengers, which can only be done with a custom airport profile, otherwise passengers would disappear as soon they go through the last step of the stairs.
That's why, the two options (disable bus and passengers walk-in paths) are together in the airport profile, because one excludes the other.