My favorite back at them, for guys that like to startle cyclist, is to ride up to the drivers side while they are stopped. If the window is up, slap it with an open hand right next to the drivers ear. If the window is down, yell in the drivers ear - something like “HELLO, seems honking and your close pass did not save you any time”.

The last guy that participated in such an exchange, had just taken a sip of something. When I slapped the window, he jumped clear out of his seat and the junk in his mouth dribbled out.