I used to know a guy who carried a bunch of smallish rocks in one of his vest pockets(the "badass" type), and when a car would tailgate him he'd flick a few over his shoulder. They weren't big or heavy enough to damage the windshield or anything, but I'm sure the produced a nasty "whack" and made the driver think the bike was kicking up gravel off the tire or something. When someone tailgates me, I just speed up and lose them, or at night let them pass, then get behind them and put on my 100 watt high beam. I can see it blasting straight into their rearview even from the bike. Usually before long they start slowing down to try and get me to pass them to get out of my death-ray headlight.
I was out the other day on a side byway, and there was some enviro-hippy in a Prius doing 15 under the limit. Behind that was a beater Civic, who was RIGHT ON the Prius' ass. Like seriously maybe a foot behind it, tagging his brakes constantly. Of course the prius rides the brakes the entire way down the slightest hill, and slows to a crawl on every corner. A narrow passing window opens up, I drop it to second and smash it, intending to pass both at once. I was keeping a comfortable distance from the civic, giving me a full second or two to build up speed to blast by the slowpoke and the slowpoke's hemorrhoid. I was wary the civic was ancy-in-the-pantsy to pass also, but there was no way the dotted line was long enough for an 80hp POS to chug up to speed so I figured I was safe. Of course, I'm wrong. The idiot in the civic(complete with fart cannon exhaust and well ventilated quarter panels) slides into the left lane to attempt a pass right as I come up on him. Of course no signal. By then I'm hitting third and already at 70 and climbing fast from our previous lethargic pace of about 35 or 40. No time to brake, so I had to pass the passing civic, we were three-wide across the road. Crapped my pants a little since I was on the narrow oncoming shoulder, and this road had poles with reflectors like 6" off the side of the road. I didn't have much wiggle room. And to top it all off, while I made it with plenty of room before the next blind corner, the civic was barely mirrors-even with the prius when the driver finally realized his POS wasn't going to make it, and apparently had just enough common sense not to drive into a blind corner in the oncoming lane, slammed the brakes and slid in behind the prius again. What a dork.