When I used to go deer hunting in the National Forest, we'd come across logging equipment all the time. They had metal plates on hinges that went over the dash that where locked into place, and also over other things including the fuel cap. I don't know if they removed anything from the machines to make them inoperable, but I do know that those locks and metal plates where pretty secure.
When I bought my land, I lived in town and I was worried about theft, so I added a hidden switch to the starter line that wasn't easy to get to and impossible to see. You had to know to put your arm into that spot and turn the switch before it would start.
I thought about removing the battery, or disconnecting it, but thought that would be too easy for a thief to figure out and then just jump start it. I also thought about adding those metal plates to the cover the key/dash area, but with so many cordless grinders out there, I figured it would be too easy for them to get past that too. Same thing with chains, it's too easy to cut them off.
I did remove one of the front tires when I first got it, but that was such a pain, that it forced me to install the hidden switch. I have no idea if it worked or not since I don't know if anybody ever tried to steal it, but it did make me feel a little better about leaving it there when I wasn't around.
