When someone asks me a question for which the answer is none of their business, my typical response is, "Why do you ask?" If and when they answer THAT question, my usual reply is, "Hmmm..interesting." And then I ignore the question.
If they pursue it further, the (still polite) response becomes, "That is really none of your concern, and I choose not to discuss it with you, thanks." Followed by walking away, if possible.