I usually turn off fuel, disconnect the seat safety switch, switch the PTO on and toggle the powerreverser forward. This would prevent drunk youngsters from starting the engine, but of course wouldn't stop anyone familiar with tractors. My only other defence is the game camera trained on the machine.
/ Leaving on vacation, ideas on disabling tractor?
On most Kubota tractors the solenoid is only energized when stopping the tractor, a few models do have a two coil solenoid using one for stop and the other for run mode.