Visual deterrents, like The Club can send amateur thieves to another car, but The Club is easy to remove so not an anti-theft device.
Also Cable thru a rear wheel around leaf spring
Fake flashing alarm LED :)
A timed fuel or spark cut off is best, but it won't stop the break in and key cylinder damage.
The timed part means thief can start engine, and drive car away but fuel or spark is cut off after 30 seconds or so.
Thief is now stuck out on a public street somewhere with a stalled vehicle, not hidden away, so they will not try to find the problem they will walk/run away.
You would set this up as automatic enable at power on, so no switch to remember to turn on/off.
You would have to push a hidden button before the 30 seconds were up or engine would stall, but pushing the button would get it running again.