Whilst digging around I took a look at the advanced options of FSUIPC and came across this :-
It only appears to work with a registered copy as the entry is already in the configuration settings with a default 'no'. When removing the registration details and restarting, the configuration settings file is somewhat smaller and the relevant entry is missing. Adding the missing entry and saving to a non-registered configuration settings file doesn't stop auto refuelling.StopAutoFuel: Set this to ‗Yes‘ on FS2002/4 to stop automatic re-fuelling at scenery fuel boxes. With this selected you can only increase fuel via the FS menu or by using a program or gauge which does it via FSUIPC‘s offsets.
So, for those of you with a registered copy and want to close one door that might otherwise give you a flight marked cheat, look for the following entry in the FSUIPC configuration settings and make sure it reads 'yes' :-
I've not checked to see if it works in FSX but I suspect not.