Learn from the stoics:
Remember that it is not he who gives abuse or blows who affronts; but the view we take of these things as insulting. When, therefore, any one provokes you, be assured that it is your own opinion which provokes you. Try, therefore, in the first place, not to be bewildered by appearances. For if you once gain time and respite, you will more easily command yourself.-Epictetus

But I usually just flip 'em the bird.