Using shear bolts, bolt some chain to the seat rail/ seat box and then weld 1 chain link to the back of the handbrake, you can then padlock the handbrake on.
Fit a valve on a solenoid that keeps the brakes locked on when the valve is shut.
Fit a kiltech, stops the ignition barrel being snapped off and the steering lock being broken off.
Do the same as the brakes with a solenoid and valve but on the fuel line.
Tracker.
Fit a coded magnetic immobiliser, similar to the on a jcb telehandler
Just a few ideas
Like the solenoid in the fuel line, was thinking of that, I have an lpg shut off solenoid somewhere that might fit.
Have thought of locking brakes but they only have to open a bleed screw and away they go.
A cut off switch on the battery is a good idea and lock the lid on the battery cover.
Working on a device to lock gearlevers apart so locking in first so it's difficult to push away.
All these will only slow them down but may make them decide to go on to the next one instead