I suspect a fairly simple software change could be developed such that the Can bus is deactivated when the vehicle is locked, or at least the parts of it which can be accessed externally.
OBD ports could be protected with a PIN / Password required to access them, this would be set at manufacture and given to the customer just like you get a code for locking wheel nuts, this would need to be an industry wide change.
All fairly simple changes which could be made if there was a desire to.