parrot can be hacked really really easily (or certainly used to be the case)
they’re an open wifi, so log in and kill the main process and it plummets. or just flood wifi deauth packets. then it’s up to the software on board to decide whether to land or fly home, cos the guy controlling it will never have control.
dji mavic, spark, etc. – dunno – do they use wifi for the link?
hobby drones, dji naza etc. all use RC 2.4GHz channel. you could get 150 local RC flying guys all on the airfield flying together, and still not exceed the limits of the RF channel. the planes/drones would drop out the sky by accidental airborne collisions before the digital control causes link drops and loss of control of any aircraft.