Setting aside that making sense of things Trump suggests is pointless: one method would be a blanket order that telecom providers turn off access in a targeted area. I expect in America some portion of people would still figure out how to get connectivity, but cutting off 80% of internet access would still be very disruptive.
Donald Trump May 'Turn Off the Internet'. How even? A thought experiment. (www.newsweek.com)
Sounds silly of course, and it is silly. But I thought it would be interesting to think it through. And with multiple minds....