Really sorry to hear about this. As has been said before, BMW need to start feeling the pain for this. We are the ones being ripped off!
They wouldn't like it if the unscrupulous youths did it to a whole dealership - not even nicking the cars just re-arranging them on the forecourt to make the point. BMW might start to take note when it starts to affect them directly.
Footnote: For legal reasons I do not condone this suggestion. Just merely made the point for effect.