The advantage to these IMO, Vigee, is that they are irreproachable in and of themselves. Totally innocent replies. But the first demands that the rude person either 'fess up or shut up (or make up something in a hurry, but it puts 'em on the spot); the second can be taken as wildly sarcastic, but there is no PROOF that it was intended that way. Thus my insistence that the facial expression and tone must be sincere and guileless. Also, in the very unlikely event that the speaker didn't understand the inappropriateness of his/her query, they have the opportunity to save face. Unlike if you respond with "Go pound sand," which has the advantage of brevity and directness, but can't really be spun as anything but what it is. 
The humorous responses suggested are less enigmatic, but also terrific.

The humorous responses suggested are less enigmatic, but also terrific.