That's not things you do to stop South Korea from installing the THAAD System, that is just your response to the South Korea installing THAAD system. You can go to trade war with South Korea or increase your aid to North Korea, to try to scare SK off from installing THAAD, but the ultimate decision would be South Korea and the United States alone, one being the willingness to install the system, the other being the willingness to sell the system.
This is the same as you can ask Cuba to install a Chinese Radar, it will be done regardless how US object or react to such act, however, the final decision is always between China and Cuba, US can do anything to object such move, but as long as Cuba willing to install the Chinese Radar, there are nothing the US can do to say otherwise.
Also, you are quite naïve to think a trade war or helping the NK would help your case, trade war does not benefit China, in fact, since China have a big export surplus to SK, it would hurt China more than SK if China is to start a Trade War with the South. Reason being it's always easier to find supplier than market, and if China export more than they import from the South Korea, that means China will lose bigger share of business than South Korea. Think of it like this, if China stop supplying IC to Samsung for their phones and electronics, For Samsung, the worse they can get is to score an IC deal with someone else with a higher price, so the end result will be raise the price of their tablet, phone or TV, and most probably, they will be able to find a comparable price to China in the like of Thailand, Philippine, Vietnam, which then won't affect their business. On the other hand, the IC company lost Samsung as the client, they will then need to find another one that buy that much in order to keep the same production level, otherwise you are talking about shutting down production and so on. Downscaling a business is always more hurtful than a slightly raise of price.
Helping North Korea more will only isolate China in world stage, and also, it will help push South Korea further to the United States then they already are, if so, anything could happen, the South Korean may even request the US to station Nuclear Missile in the south just in case of a Korean War do-over.