Well it depends by what you mean by a kill switch. In the UK there have been instances where large parts of the UK have been without Internet services simply due to theft (some router cards being stolen in Docklands in London) and accidental fire (one in Manchester in an old BT tunnel linking some nuclear shelter bunkers for the secret 'Manchester Guardian' telephone exchange). Superusers can remotely kill the Internet connections for millions of users in the UK by logging on here;

www.btwholesale.com

The US ISPs won't be any different.

The easiest way to cut off millions of customers is just to gain access to the ISP customers billing database.

The fat pipes of any fibre network can be disconnected rather easily with a leatherman tool.

Taking down the internet is actually rather easy.