China may well be able to exert control over the South China Sea. At the very least, it would be extremely hazardous for the US Navy to send any surface ships into the area.
BUT
The US submarine fleet is the best and most advanced in the world. It could certainly deny the use of the South China Sea to China by torpedoing anything Chinese that entered the area.
In the event of such a conflict, the US Navy could and would impose a naval blockade on China. It would be quite effective because so much of the energy China needs still has to cross through the strait of Malacca and the Chinese navy doesn’t have the legs to effectively project power that far - and of course US attack submarines which could sink any oil tanker trying to make it through to China.
Of course, this may well change in future years as Russia builds out the pipelines to China needed to supply them with oil and natural gas thanks to the Deep State’s idiotic decision to push Russia straight into China’s arms.