How sneaky do you want to be... and how much trouble are you willing to go to to get your tractor in runnable conditon again when you want to use it. I break down the security issue as follows... the more of these you implement, the harder it is for somebody to get the tractor... and the harder for you to get it running again.
Fix it so it won't start:
no key in it
change out key lock to a non standard one
remove battery
insert battery cutout switch and key... $6 at harbor Freight
jigger the electronics so it thinks it is in an unstartable condition such as.. PTO ON, CLUTCH NOT ENGAGED, SEAT NOT OCCUPIED
shut off diesel valve
run dry... put air in the fuel lines
bobby trap it with a siren/tape recorder that will sound off when it recognizes that the tractor is being tampered with in an unauthorized way. Tape recorder should say that the local police/sheriff has been notified by radio that a theft is in progress and they have said they will be there in 5 minutes. (we know this will not be true.. but will the thieves?)
When I was married, my best man booby traped our car at the motel... when we came out the next morning and started the car, there was an amazing whistle sound that got increasingly shrill and ended in a huge puff of smoke from under the hood... some sort of "firecracker" that was set off by hitting the ignition.... certainly got the attention of all in the area and might scare away mischief makers.... I don't know if such a device is still available.
Fix it so it is difficult/impossible to pull up on a trailer
fluid in tires makes it heavier
remove front wheels/tires
chains thru holes in front wheels, chain them together
chains thru holes in rear wheels, chain them together
park in location, if possible, where a tow vehicle can't get to it... a gully... between trees, extreme slope for tow vehicle, etc.
extreme weight on ground engaging implement, dig it in deep, 3ph max down, locks thru 3ph pin holes, not lynch pins.
locks thru 3ph link arms... not the pins it comes with
set brakes really hard and remove orange handle and decals indicating how to set the brakes.
If you have a FEL, engage it into ground so it lifts tractor off ground almost all the way... but not obviously... use the FEL travel lock to prevent relieving hyd pressure with joystick... this is hard to figure out how to engage/disengage, even with the manual.