Just pull the power cables and disable the UPSs from a couple of hundred RAS's and Gateway Servers throughout the USA (even easier with Super User access to the ISPs RAS and Gateway Server databases) and pull the fibre router cards that service the submarine cables in and out of the US. No Internet services for the subscription clients for internet service providers whether this is 3G, Edge, DSL, Fibre etc and no fat pipes into and out of the USA. You might be able to get a dial up connection to an ISP in Canada or Mexico, but most likely in this scenario ordinary telecoms would be blacked out as well. A well planned exercise in shutting down the appropriate fat pipe fibre nodes, rerouting government and military TCP/IP traffic on a shadow network and terminated all the ISPs customers PPP sessions and not allowing them back on to the ISPs networks (lack of PPP authentication access) could probably be achieved within 24hrs

What is even more amazing is this;