You Left Your Blinker On, Idiot

How do you communicate to someone on the road that their turn signal keeps flashing for no good reason? If you beep at them, they don't know why you're honking. I've tried beeping and getting their attention and making some hand gesture but they probably thought I was some pervert. A few times, I've moved in front of the offending vehicle and started sympathetically blinking, but they didn't get it (and neither did I when someone did it to me). So, I've decided there needs to be some universally accepted sign to commuicate that "You left your blinker on." Whether we should decide on a hand signal, or maybe a special honk (how about three short beeps? "bip-bip-bip"?) , there needs to be some simple and easy way to say "Hey, dude; your signal's on."