How about some of that plastic lifelike dog poo on the drivers seat?.... Would that put them off?? ....no.... yes??

Seriously though, it's got to be something either very obvious that would mean they'd have too much trouble to overcome - not something that just needs unlocking, because they'd get THAT key with the car keys! - or something unseen like the remote fuel pump switch............ On some of my 'older' cars I used to insert a hidden switch to the distributer wiring, no amount of engine turning would start it!