Country A can put an embargo on certain products to Country B. When Country C sends those products to Country B, Country A refuses to do business with Country C, or fines it, or levies some other penalty.
Country A is still involved in their transactions with country C. The only way they are exercising any power is through their direct involvement in the trade network. This also isn't a legal court case with a crime and jurisdiction so it doesnt even really fit the idea of where crimes are evaluated in a court of law - international trade wars are not criminal law unless there are crimes against humanity involved, which I already put a disclaimer about as one of those crimes considered having universal jurisdiction. Though you still have to invade them to be able to prosecute people, in which case you extend your jurisdiction by turning the country into a territory as in ww2
