Look them straight in the face while saying: "I can't believe you think that is ANY of your business." Then walk away. Calmly and directly, shut them straight down.
Of course the Silent Glare of Imminent Demise is a good one to employ too.
I said something like this today to the scheduler at my doctor's office. She asked the question when I said I might have to call and reschedule an appointment I was making because I'm waiting to hear from the children's hospital for a day and time for surgery on my youngest son's wrist. Thank you to everybody for validating my desires to be less than "smiley" in these situations. It really made me far more comfortable in dealing with it.
Wait, a doctor's employee asked you this???
Yes, and I love my doctor, so even though this office staff member did this, I'll stay with him, but I will mention it to him the next time I see him. The conversations went something like this:
Staff Member: Is [date and time] okay?
Me: I think so, but I'm waiting on surgery scheduling from Seattle Children's to call me with a surgery date for my son. If I have to reschedule, I'll call back.
Staff Member: Oh, you're the one with all the sick kids, right?
Me (completely floored): Well, I do have to kids with a bone condition.
Staff Member: It's genetic right? I always wondered why people would choose to have kids when they could be born so sick.
Me (Icy stare): Well, that's a rather personal decision, isn't it?
So, I did my best with the icy stare. I don't know if it was good enough, but the rest of the conversation was that uncomfortable "polite" feeling you get when one person knows they've crossed a line, but you still have to finish whatever it was you were doing.