A friend of mine had a late 80's Jeep Cherokee that he disconnected the washer fluid hose from the rear wiper arm. All he had to do was push the rear washer button on the dash and it squirted right on the offending tailgater's windshield. Worked every time.

Ive considered getting a magnet sticker saying:
"Driving slower to save gas/$$$$
Feel free to pass"