South China Sea, the "second Persian Gulf" - What if China shuts it down?

South China Sea navigation routes
(Eighty per cent of Japanese and 39 per cent of Chinese oil imports pass through the Indian Ocean en route from the Middle East)

China has fired off plenty of propaganda, along with a few volleys from People's Liberation Army Navy ships, in response to the international court ruling on the South China Sea, which it denounced as "null and void". But after threatening to set up an air defense identification zone, what might the economic impact be if Beijing went one step further and closed off the entire area inside its so-called "nine-dash line"?