This is nothing new. Crazy people can be anywhere; they can follow you around, or simply be at your location. The Internet doesn't make this different, it just makes it slightly easier to target a specific individual. But Foursquare, unlike "real life", you can control. You can fake check-ins. You can simply not check in. But when someone is following you around in real life, there is nothing you can do, except maybe get a fake mustache.