Just turn them off and on- It makes it more painful for the person. Why?
Your cornea contracts according to light- So when it's dark, your pupils gets bigger to allow more light in and contracts when there is a lot of light, just like Aperture in Cameras.
So, when you're behind an idiot who just overtook you by flashing his HID's like a crazy maniac, get behind him and keep flashing at intervals for over a 1 sec- Just enough time for the Pupil to start expanding due to less light, then contracting again because of your HID's. Payback time Biches.