There is no key lock on the inside to lock the trunk release? Could one be added. Every Toyota or Honda I have been in since 1985 has had a key lock for the trunk, Either buy turning the trunk key the opposite direction or by locking the release inside the car.
Try lubing the lock assembly and see if it will lock in the opposite direction to unlock. This prevents inside release from activating the release. Check dealer to see if this is an option.