Most people have been in the somewhat irritating situation of talking to someone who is prying into personal matters that are simply none of his concern. While this can come off as singularly obnoxious, it is important not to assume that the person is being purposefully rude; he simply might not realise that he is being inappropriate, or he could think he is much closer to you than he actually is.
Consequently, it is best to politely tell someone who is overstepping her bounds that she should mind her own business.
Look the individual in the eye. Eye contact is an important tool in interpersonal communications. When you first meet people, looking them in the eyes is the polite way to acknowledge them. After that, periodic eye contact lets the person know you are still paying attention to him, but when you accompany a particular phrase or comment with direct eye contact, it is a way of emphasising what you are saying.
Address the individual by her name. This is another social convention that is expected when you first meet someone, but it has the effect of calling special attention to what you tell the person following the direct use of her name.
Lead with a quick mention of how you appreciate his concern in the matter. This will have the effect of a final courtesy to soften the connotations that surround cutting him off from his particular line of inquiry.
Tell the individual that the issue is not her concern, so she shouldn't worry about it.
Smile and ask for some sort of confirmation that he understands, such as "do you understand?" or "all right?"