Let me explain again. There is a risk of spoofing in relation to ADS-B because once you remove the expensive rotating antenna as used for primary and secondary surveillance radar, the only confirmation of the position in a simple ADS-B system, is what is transmitted by the sender. This position can be false. One way around the spoofing, is to have a system where there are a number of ADS-B receivers and they compare the position transmitted with the time intervals to each receiver.
True, the position could be false... but don't forget that it is the responsibility of every controller to correctly
identify an aircraft symbol on any surveillance display in accordance with a long list of rules that are universal. It is then the responsibility of the controller to
verify that the information transmitted and displayed in addition to the symbol is true and correct. ADS-B makes it significantly easier to achieve this, but doesn't remove the requirement for identification and verification.
As for unlawful transmission of ADS-B information... every working day using RADAR I have to sort out the legitimate RADAR symbols from the Angels, Ghosts, Reflections and Garbling (all terms that we use to describe common anomalies displayed on the RADAR).
As for the radio transmissions, I hear the same voices, see the same aircraft, process the same flights. You become very attuned to what is real and what is not. That is how both pilots and controllers ascertain that some idiot with a hand-held radio is attempting to play pilot or controller. It stands out like canine genitalia.
When an itinerant flight comes along, it attracts a different sort of attention... a search for, and full scan of the flight plan... identification of the aircraft by more than one means... an informal assessment of the pilot's English comprehension skills and apparent competency in receiving and complying with ATC instructions.
We know to expect the flight because of flight notifications and approvals received... otherwise a friendly chat with my military liaison officer ensues and the matter is then in his hands.