|With regard to the original question..............|
If your internet is connected to a multihomed PC, you could use the netsh command to disable the network interface that connects to the internet. Once you've figured out the correct command syntax to do the job, you could then create two batchiles, one that would disable the NIC, and a second to re-enable it. A good batchfiler would set it up so the batchfiles would actually ask which interface to dis/en-able.
If you use a SOHO Router then you're probably out of luck because the Manufacturer would have to incorporate the "kill" feature into their hardware.
If you're using an enterprise level router, then you could login to your router and manually disable/re-enable the appropriate interface, but I doubt you could automate it. It may be possible to write a batchfile to do the job but it would require you to first login to the switch, change the correct command level, and then initiate the command.
To sum up, other than my first suggestion regarding a multihomed PC, the most likely reason nobody has invented a "kill switch" like you're suggesting is because they can't.
I'd be VERY curious as to why anybody would want to do this. Care to share that information with us?
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.