Trick I am looking for is secrecy. Removing a battery or letting air of tires is obvious and easy, but a hidden kill switch requires more IQ than thieves have. Just need to know where to put it so it works without being a pain during normal op.
Unfortunately with it being a diesel the ignition really doesn't mean anything, all they have to do is jump the starter so it will crank and use some starting fluid to get around the glow plugs.
Even adding something to cut the fuel wouldn't stop them if they really want it, a trailer with a winch and they can have it loaded and be on the road in a matter of minutes.
Your best bet is to always park it in a place where it would be harder to get to it, and as much out of sight as possible, park another vechicle behind it while it is nosed up to a building or big tree or something.
Another thing to consider, if they really want to steal it and you have made that impossible, they could get rather upset and do alot of expensive damage.
3 months after we bought our farm we had our backhoe sitting out here and we were only gone for 48 hours but when we came back out they had busted all the windows out except the windshield, but they cracked it up pretty bad, put holes in all 4 tires, and dumped dirt in the fuel tank.
I know this isn't the kind of stuff you wanted to hear, but unfortunately that is the way things are these days.
Using a camera to help identify someone messing with it would be your best bet, yea it won't stop them but maybe you could catch them and maybe even make them pay for it.
Insurance would also be a good thing to have, but make sure you are covered if it is remote, and unattended.
Best Regards
Paul