See it all depends i think what they are being Pretentious about. I read an answer to this question that said " and you are ?" well what if the guy says i am so and so and that just happens to be a really really important person. It do not give the right to be Pretentious but the "and you are" answer is simply wrong.
if the person is Pretentious but knows what the hell he or she is talking about listen. you might not agree but thats ok. By listening you will find arguments to trap him or her. the more people talk the more they give you ammunitions.
Just wait for him or her to make a mistake in his logic and jump in for the kill. He or she will never suspect that such a polite person as you will eventually put them back in their humble spot.