Nothing can or should be done!!!
A multi day Disney ticket entitles the holder to use of the Disney Transportation system. Staying on site does not get you any advantage there.
As for the "off site" guests overcrowding the buses? Where did you see that? I really doubt it's a fact, just one of the DIS theories.
I fail to see how it's "problematic" at DTD since Disney buses don't go from ANY parks to DTD. Years ago they did and it was an issue since folks tried to "park for free" but Disney wised up.