You can remove all Vehicle parking spots. That will eliminate the fuel trucks. You can also just move the ground vehicles slider to ZERO.
If the same behavior continues after doing either of those, it is not the fuel truck.
As for the arriving planes, the fuel truck is part of the shutdown phase of the flight. After the fuel truck departs, the plane goes in to SLEEP mode.
It only starts the Pre-Flight mode when the AI flight schedule calls for the plane to next takeoff.
Look at one just arrived aircraft? What is its UTC time scheduled for it to depart? Does the plane pass that time without going into Pre-Flight mode? That should begin in the minute after the aircraft scheduled departure time.
I use the TrafficToolbox.dll Explorer utility to view the exact state of AI aircraft. It is part of the SDK.
@ PawPaw's house - near KADS, Addison, Texas, USA