Several states are beefing up their restraining orders on potential girl-friend beaters by making them wear global positioning system (GPS) bracelets. When these guys get within a restricted zone, such as near the potential victim's house or work, she gets a quick warning message.
In the early studies, none of the bracelet wearers has violated the order to stay away. The researchers think that this excellent result is due, in part, to the restrained guys knowing that someone is watching them, that the object of their obsession is not alone in the world.
This seems to me to be a good thing. The barrier so far is cost -- about $10 per day. I think this cost could be driven down dramatically through further automation. And this is the sort of ounce of prevention that pays for itself in avoided law-enforcement costs, pain, and death.