I have seen videos of a person using his belt as a leash to tether a dead OW back home. Not a bad idea that fortunately I have not had to do.
As for a kill switch one has to remember that an unpowered OW rolls a lot farther and faster than a powered OW with no rider on it. Test it for yourself and you will see that when the OW thinks there is no rider on it, the hub motor applies the brakes so it doesn't roll away.
As for a potential liability when a runaway OW torpedoes an innocent pedestrian, I make sure I am going really slowly when I am near people. Going at a quick pace near people is asking for trouble.