Traffic cops say if you know someone is being agressive, avoid eye contact as if you can't see them, look elsewhere. Then they think that anything that happened was not personally directed at them and see it as a mistake (not that you've made one!). Just give it the 'not interested' treatment.
Yep, be strong, keep calm, keep focussed on the road and stay driving at the speed you are comfortable with. Don't give in to the pressure. Some miles down the road, you might see the ignorant motorist in a ditch somewhere.
If you are actually faced with the person and can rise above the situation ( as far as pride goes) then take the wind right out of their sails by apologising profusely, grovelling and generally taking all the blame. It's no skin off your nose (so to speak) and may actually save it getting broken! If that's how they like to behave then let them carry on and you will have the admittedly dubious satisfaction of knowing that they are a**es and you are not! It does work - I've had to do it more than once as I drive for a living!