Sailfish2, China and Russia would definitely have to be on board with a "quarantine" of North Korea because they both have land borders with North Korea.  However, China and Russia have much more to gain by their relationships with the United States and the civilized world than by supporting a single rogue regime like North Korea.  And thank you for the information about blockades, which I already knew.  President Kennedy also knew that a blockade was an act of war which is why he imposed a “quarantine” on Cuba to force the Soviet Union (Russia) to remove missile capable of hitting the United States and to dismantle the missile sites.  It worked.