"Smile and wave" isn't a bad suggestion. I would say just take a deep breadth and ride on. Assume the individual has a low IQ, smile to yourself at that realization and continue as you would normally. Need reason to let some ******* take away from the enjoyment of your ride (unless he/she is a real a**hole and continues to harass you. Then you find an alternate route ASAP).
Be smart. Be safe. Always have a plan b, c, d, etc.....