Leave them alone.  That’s the People’s Republic of China’s reaction to the US/Republic of Korea agreement to station an anti-missile defense system in the RoK, a setup explicitly designed for defense and deployed in response to northern Korea’s missile and nuclear warhead buildup.

PRC’s Foreign Ministry spokesman Lu Kang:

We demand the US and South Korea change their attitude.  By agreeing with the US side to deploy this system, South Korea has gotten itself involved in a situation that damages the current balance on the peninsula.

Yeah—the balance the PRC and their client were trying to develop, from which the better to intimidate the RoK and other regional nations.  Because defensive systems are threats, but PRC/northern Korea offensive systems cannot be.

Lu also refused to rule out economic retaliatory measures against the impertinent RoK.  Roughly 25% of the RoK’s exports go to the PRC.  This comes, also, with the PRC’s history of economic retaliation, as it did against Japan when the Japanese successfully disputed the PRC’s attempt to seize Japan’s Senkaku Island group.