Blazing Cat Fur
The US govt. licenses broadcast stations under the principle that the airwaves are owned by the public, not private interests. If a broadcast license is granted to a local station, they will have regulations to follow. At one time (maybe still), broadcasters were required to provide a public service, typically in the form of news. BS is usually not a public service.
I don’t think Trump can completely shut down the major networks, but he can deny licenses to local stations that air BS from major networks.