What if I didn't need a refuel?
Just don't call it!
All I did was hit "prepare for pushback" after boarding, when I was ready to go, and it wants to send me a catering truck and fuel truck? I don't need these things.
Have you maybe enabled the "Assistance Services “Auto” mode" option in the Settings ? Quoting from the manual:
Assistance Services “Auto” mode
This option makes GSX running on Autopilot, activating all its services in sequence, automatically. If you start from Deboarding or Boarding, the autopilot will call Cargo/Baggages services first, then Refueling and Catering at the same time.
If you enabled it, it's like you said to GSX you always need these things, in that particular sequence. Lots of users asked for it, so we added that option years ago, but it's Disabled by default.
If that option is not enabled instead, I'm not sure if the Maddog integration calls for Catering or Refueling automatically but, if you had the "detect custom aircraft system refueling Enabled and then you manually loaded the fuel, the Refuel truck was triggered by that, because that's precisely what that option does: it calls Refueling as soon it detects an increase in the Fuel quantity.
As I've said, the one and only reason for that option to exists ( and the reason why it's Disabled ), is to prevent GSX "missing out" the refueling of an airplane that starts progressively refueling itself automatically after some time, it was a very fringe case that only a couple of airplanes in P3D, like the PMDG 777, used, but it should normally be Disabled, as it is by default.