Once the Doctor's companion started carrying this, every episode was two minutes long.
The DroneDefender disrupts the drone's control signal by bombarding it with ... well, even more signal. It's the electromagnetic equivalent of a Skrillex concert when someone over 50 is trying to talk on the phone: They're both using the same technology, but the victim has no choice but to stand paralyzed and deafened as they try to comprehend the overpowering noise.
Of course, this "fire so much signal that everything goes electronically deaf" means the DroneDefender has to tackle a far more terrifying foe than the drones: the Federal Communications Commission. Which reportedly hasn't stopped the Department of Defense and Homeland Security from placing large orders. I suspect they're only selling to the federal government (so far) because they're the only ones who can afford the legal fees resulting from firing this anywhere on the continent.
Battelle via MilitaryTimes.com