richms: unidirectional screws sorted it for a friend, till his whole bumper was nicked.
And the process to get replacement plates needs to be tightened up.
I recall seeing a material on TV that was a clear acrylic type (such as European car plates are made of) and if you pulled it too hard so it bent beyond a certain point (as you might when trying to steal a well fixed plate) it broke something layered in it and it just went black, rendering the plate useless because you could no longer see the numbers.