I'm afraid this incident is going to lead to the end of aviation as we know it.
Pilots are happily following the instructions of a small child on the radio, what reaction would they have to someone "hijacking" the frequency and giving all sorts of plausible instructions, all designed to make them crash into each other? There is no way to verify the authenticity of ATC instructions, the potential consequences are horrific.
The only solution as I see it is to get the instructions written down, with an authentication code, sealed in an envelope and then couriered to the aircraft. The pilots check the authentication code and then comply with the instruction. It's going to be tough, but what other choice do we have?
I'm sure that the same company that provide the security search staff could provide a large stock of intelligent, reasonable, reliable couriers.
Max