It is impossible because North Korea has borders with China and Russia both of which are really not pro-U.S.  They would never allow U.S. troops into their country to block the borders.  And, is the U.S. prepared to shoot ships and planes that may be arriving from China, Russia, Iran, etc.??  Is the U.S. prepared to shoot North Korean ships and planes??  Shooting them would definitely be an act of war.

A blockade is an act of war - see http://www.globalresearch.ca/blockades-acts-of-war/9866
Under international and US law, blockades are acts of war and variously defined as:
— surrounding a nation or objective with hostile forces;
— measures to isolate an enemy;
— encirclement and besieging;
— preventing the passage in or out of supplies, military forces or aid in time of or as an act of war; and
— an act of naval warfare to block access to an enemy’s coastline and deny entry to all vessels and aircraft.