Generally when vehicles are repossessed, a tow truck turns up, either lifts it onto a flatbed or lifts the driven wheels off the road and off goes your vehicle.
Far easier than arguing with the owner, takes literally two minutes and I've seen many cars removed in this fashion (both legitimately and theft).
Even disconnecting the starter will not stop a lot of people as you only have to put the ignition on, pop a screwdriver on the starter feed to solenoid terminal and job done. This is assuming it's an automatic and you're not able to bump start it.
Better off killing the power to the engine ECU / Fuel pump / Both - possibly replace the fuse for these with a blown fuse when the vehicle is not in use ?
You're better off with a decent CCTV and/or making sure you legally own the vehicle so if he takes it, you can prosecute for theft. Another option is to just get the immobiliser reprogrammed at a dealer with a new key and the old keys wiped - no valid key means you can't drive off in it.