If you were wondering ................

So just how do you turn off the gas supply to a country.

Well take a look at Slate magazine's explanation

Switch off valves in the pipeline or shut down the compressor stations. Gas pipelines typically have valves spaced out every several miles, enabling gas to be shut off if necessary. In addition to regulating how much gas is being delivered downstream, the valves are there for safety purposes, enabling pipeline operators to stop the flow of gas immediately if there is a leak, for example. The valves also allow operators to shut down a section of the pipeline when it needs maintenance. Depending on the pipeline, a valve might be closed manually, or operators may be able to push a button that mechanically shuts the valve