Take the tires off and put them in the bed. Nobody likes messing with them heavy things. The couple of people I've asked to help just turned around and RUNNOFT...
Seriously though, just remove the label on the dash for the battery and start switches. Unless they know these trucks, the switches just look like the rest. If they do know these trucks, you didn't have a chance anyways.
Think about it like this:
No PCB = no start
No fuel = no start
No air pressure = no go
There's a couple of low amperage wires going to the PCB, where a simple toggle switch could be put in line to keep the solenoids from engaging.
The fuel shut-off solenoid is operated by a single low amperage wire. A simple toggle switch in line would do the trick.
A simple toggle switch in line with the neutral safety switch will work too.
Leave the tank drain valves open. The brakes can't be released without air pressure and you can't hear the air blowing if they're all open. The tanks are supposed to be drained daily anyways, right?
Leave the truck in reverse and transfer case in neutral. No start and no go. They'll have to figure that out, along with the battery and start switches, and the drain valves.
There are many things on the M939 trucks that can be done without modification, to create a no start situation, that would leave even the seasoned owners scratching their heads.
I spent a couple of hours one night trying to figure out why my truck wouldn't start after replacing some gauges. I thought a wire had come loose behind the dash. Turned out I had bumped the shifter into 1-5. Was the last thing I thought to check.
There isn't really anything in the cab I can't do without. I have the locking door handles but don't use them. I'd rather somebody just open the door then break things trying to get in.
Don't over think it. Simple will do just fine.