I used to have a little fat man doll stuck in the back window. Attached to him was a tube and a plastic bulb. Squeeze the bulb and he leaned forward and dropped his pants.
I felt better for it and it got the message across, quite often causing a smile.
I also had (and still have it in Turkey). A stick on notice "If a**holes could fly this place would be an airport".
You could also use the "Avenger" that sits on the dash with buttons to operate the noises of machine gun, howitzer and death ray.
It helps to get it out of the system