What do you think when someone judges another person's every move?

Just tell them "You know what's pathetic? You have NOTHING else to do with your time other than to stare at me. Get a boyfriend or something. Get a hobby. Get a drug addiction, just get SOMETHING" and then go back to what you were doing. If they try to engage conversation with you, don't give them the time of day. If it doesn't stop, sue them for harment. I'm not kidding. If there's anything that will get people to leave you alone it's legal action, especially when money is at stake.