Get a couple of small slave flash guns from your local photography shop. These are the ones that trigger from the light of the main flash going off and are meant to provide "fill-in" flash for photgraphy.
Put one in the front window, one in the back and when the flash goes off they will reply in kind. This will hopelessly over expose the image and the camera cant see a thing.
This idea was dreamt up by some celeb's minders and was first used to frustrate papparazzi trying to take photos of people getting in and out of limos. (The guns are let into the door panels)
It was tested a couple of years ago by What Car" and it works a treat.
Of course, it's illegal.
And it won't work on the new fangled cameras that don't take flash photos.