Start by staring back. Usually direct eye contact will break the stranger's total fascination with gazing at your face. If that doesn't work, proceed to Step 2.
Speak. You can be direct and ask, "Why are you staring at me?" or try a humorous approach. Some may include: "Wow, you must be staring because I am so beautiful," "How nice of you to notice I got a brand new hairdo" or "Are you staring because you're a Hollywood talent scout and I'm the next Marilyn Monroe?"