How do you bring a bad drone down? New kinds of drones that can fly autonomously can't be stopped with traditional techniques, the US Air Force has warned. It's put out a call for ideas to yank drones right out of the sky. Millions of drones are sold worldwide each year. Most are flown for fun, but a few have been put to criminal use: carrying cameras to bedroom windows, flying into secure airspace over nuclear power stations, and smuggling contraband into prisons.