Nothing to be done? True ... except maybe take down a license number (sure, that'll help).
Well, there is a bright side
There is a kernal of an idea for defending yourself against the tail-gator (relative of the alley-gator ;o). What you need is the Ebtx intimidator ... based on the afforementioned bouncing debris.
Observe the miraculous design in action
OK ... this thing just shoots BBs ... not rocks. The greatest asset here is that a BB travelling at about 40 mph will not damage the other windshield but WILL make a God-awful sound inside the other car (enough to cause an immediate breaking action on the part of the offending driver). They have to slow down to back off in order to save their car from scratches & dents.
It will work ... I guarantee it.
The other driver will think that your car has thrown up a stone. He won't guess that you activated the mechanism since you won't give yourself away. All your actions are hidden from view. The only way he could know is if he saw your face ... but then ... he has to slow down while you are laughing in the other direction. Hah!