Get a really bright flashlight and then when they don't take any hints, roll down the window, aim that sucker backwards and let those photons rip. Just kidding, although I have thought about it, I have plenty of very bright flashlights (thanks r/flashlight). There's no good way to do it, I usually just try to lean out of my mirrors way and avoid the light, they usually turn off soon enough. I've tried brake tapping and it gets them to back off, which is great but not what I wanted (they weren't tailgating anyway), and while the hazards could work, they'd probably just think "oh, they're having car troubles." I wish car's had an LED readout on the back or roof that could be programmed to say what you want, like "hey, turn your brights off"