Gravel driveways are good for deterring people as they are really noisy to walk on, downside is that they are really messy and your neighbours will hate you.
Can you install an electric fence? Get cattle prods, I am sure they will do the trick
Maybe a little OTT, but in the James Bond film when the car was set to blow up when broken into, that seemed quite successful, they won't try that again in a hurry, or much of anything else to be honest.
You could fill the car with "dangerous" animals, so anyone breaking into them would get savaged, although the cleaning bill for the cloth / leather after the odd accident may mount up over time.
Apart from that, moving looks like the best option
Find properties to buy, homes to rent, overseas houses and new homes on rightmove.co.uk
Erk - I typed www in before the rightmove web address and it filled all the extra gubbins for me
