How to prevent someone taking my vehicle if they have a key

Got power but car wont start

Three possible ways I can think of: (Disclaimer; I'm no mechanic) 1) Disconnect the battery. 2) Remove the fuse for the ignition / ECM (computer). Determine potential side-effects BEFORE you try this, check your manual, find the correct fuse for the ECM and pull-it or run a 'secret switch' from that curcuit to effectively disable the vehicle when required. This will also shut-down a vehicle that won't turn off due to something like a defective ignition switch. WARNING! Be careful, do your brand research and consider the fact that you'll likely lose your clock settings (at least), possibly disrupt your security system and can even potentially lock yourself out of your own security system, similar to removing your battery in some vehicles. 3) Install a security system. Even a cheap mickey- mouse system that requires a fob or code to start. (better to have it installed by a professional) DO NOT under any circumstances, try this with any BMW product - ever!

They probably also track your car so you can't hide it.

I get the Repo thing, for practical purposes they're virtually un-stoppable. I was thinking the question related more to an armed services member or worker leaving town, possibly an 'ex' or a pesky friend/ family member that just won't give-up... which, come to think of it, the latter are probably just as determined as the Repo guy.