There are many ways to do this. You can put a cutoff switch on the battery cable, the hot wire to the key switch, the hot wire to the injection pump or in the wire going to activate the starter. Any one of these methods are effective as long as you can place the switch so it can't be easily found. You could just get in the habit of removing the fuse to the key switch also. This can be just as effective in deterring theft.
There are several safety interlocks on my kubota - clutch, pto, the rocker pedal for forward/backward. I put the pto control on, then unplug the clutch switch and hide the wires. that makes two "kill switches" that would take a while to puzzle out. also park the thing in the middle of some trees so it cant be hauled out by a tow truck.
Seeing Kubota uses one key,not much of a deterrent.Battery switch may be the easiest to do,lots of people do it for their"classic"cars that are in storage.