I respond to horn blasts by responding (as calm as I can muster) as if asking what they wanted my attention for. Since I'm not starting with an attitude and not even recognizing their actions as challenging, they can only be so much of an a-hole as they take far more time explaining to you their gripe (which they usually know deep down is unfounded) than they would have just letting you go about your way. I then usually respond with a simple "Oh" and get out of their way, usually too late to make a difference for their hurried taste anyway.

If I feel threatened in some way, I'll take a picture of the license plate. That worked like a charm getting this guy to stop buzzing me on my morning commute... around 6:00am on a road with two lanes in each direction and no other traffic around... in his self righteous Prius.