MG23: I was at the ICAO meeting when the MoU was signed between the US and ICAO at the end of the 90's. the clause was there.
Obviously they can shut GPS down. Equally obviously, it would be a disaster as many essential IT services around the world that use it for precision timing shut down within minutes to hours.
And pointless now there are a couple more systems that will work just as well as GPS for targeting bombs.