Short of a thermonuclear EMP pulse or shooting down the communications satellites, they can't. Radio and television stations go along with the Emergency Alert System. It's not the government actually shutting them off, which, short of the above mentioned scenarios, they physically can't do.
The Internet is a different story. Strange as it sounds, the Internet is the creation of the Department of Defense, and they can turn it off like a switch. Radio and television? Not.
It is my understanding that all broadcast stations have been retro-fitted with a control box interface, hard wired to the regional EAS system, so the EAS has priority to override. Any attempt to remove or defeat the override renders the equipment non-functional. It is no longer in the station engineers control at all. Any group attempting to seize control and get a message out will have non-functioning equipment.
This has been done for several years now and has been posted on FR. When the time comes to keep Americans from communicating with each other, it can be done. Any radio transmissions from Amateur Radio Operators or CBers will be dealt with as needed. You can’t jam an entire continent - I think...
Dear Broadcast professional,
Your government is conducting a test of the national Emergency Alert System (EAS) on November 9, 2011.
All television and radio communications must be turned off at 2:00PM EST. Per executive order any station continuing to broadcast between 2:00PM EST and 2:10PM EST will be subject to an immediate and permanent revocation of its privileges to use the public airways.
We are looking forward to your full voluntary cooperation during this critical test.
Sincerely,
Your Betters at the Department of Homeland Security (DHS) through FEMA, the Federal Communications Commission (FCC), and the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS).