A few years ago, I recall seeing a license plate cover that was purported to be able to defeat the camera at toll booths etc. As I remember, it was a clear cover with some kind of prismatic grooves which enable the plate to be read from directly behind at up to about a ten to fifteen degree angle downwards, so that law enforcement personnel in vehicles would be able to read the plate without noticing anything out of the ordinary. However, the grooves (or whatever the technology was) would make the plate unable to be read from an angle over 10 degrees, making a camera mounted up high useless.
Of course, such a thing would probably be illegal and I would never recommend that someone pursue it, the above is strictly for informational and entertainment purposes, your actual mileage may vary, blah, blah, blah....
Does not work...saw a show about this on local fox NYC news couple of nights ago. They tested it. looks like an infra red based camera that is unaffected by prisms or that spray stuff.