There are a few, old fashioned types, who were taught to honk when overtaking or to alert someone when they approached from behind. Some with a more country background will honk when they recognize you. But most I think are trying to intimidate you, to bully you out of the way. You can't always tell which it is.
I just give a wave, no finger even if I'm imagining it there. If I think about it I'll look around in an exaggerated manner, try a goofy grin and a big old wave and the payoff is that look of confusion (imagine them thinking, does this guy know me? Recognize me?). Since my primary rule is "don't engage the crazies", whatever action we decide on shouldn't be the one they expect. Anger, fear, obedience, confrontation, anything like that invites them to continue. Acting stupid or oblivious takes it all away from them.