A simple smile or wave to the person "flipping the bird" is probably your best bet. Gone are the days when I felt the need to flip someone off while driving, but I truly can't tolerate the brake checks or lane blocking. That sort of childish action could lead to some serious road rage that can get both drivers in a situation they don't want to be in.
Oh, never flash a gun, the other guy might have one too and use it.....at least here in Arizona. (the wild west)