The key (pun intended) is to do something nobody expects, or can see. Back in the 70s when cars were cars and not rolling computers with motors, we would do something simple like disrupt the 12V to the fuel pump. A 1/4" headphone jack mounted someplace convenient looked inconspicuous enough, but no one would suspect that the car wouldn't start without a headphone plug inserted, even if the plug was laying right there in the console or ashtray (remember those?).
We might have done the same thing with a magnetic door contact for alarms. The magnet could be stored right there on the dash (when they were still metal). Move it two inches to the side to engage or disengage the magnetic switch.
If a scumbag knew it was there, they could bypass it easily enough, but they don't want to spend time looking around for oddball things like that.
I'm not sure how the systems are wired on gasoline, propane or battery ATVs, but there may be a similar simple point where 12v could be disrupted. For a diesel, you might be able to rig something with the fuel shutoff solenoid.