Can you reverse it back to almost touch a wall (if a hatch back) this may even work for a saloon if you can get close enough? then without moving the car, there is no way to open it!??
Any way you can padlock or chain the boot shut from inside the car? get a torch, chain, padlock, let down the back seats to access the boot space and crawl in and chain the boot from the inside? maybe connect it down to the spare wheel compartment? Assuming your spare wheel is secured by bolts in the boot space?