I don't agree with bad road practice for bikes or cars, but any sort of banging or slapping has to be surely classed as road rage?? All this will do is short term is encourage the driver to go into a fit of road rage and cause danger to himself and the cyclist, and in the long term it will make them have a hatred for all cyclists on the road and be far more likely to treat them like crap and cut them up everytime they go past them.
I for one would not be inmpressed if a cyclist banging his fist onto my car, not sure I would be able to just let it go either.