NK is holding Seoul hostage. There's a reason nothing has been done, and it's largely because nothing can be done.
In the event of an outbreak of hostilities, every single military wargame scenario I've heard has Seoul getting leveled to the ground, or at least heavily hit. The economic, cultural center and most densily populated area of the entire peninsula. It would be an unmitigated disaster for SK. And as others have pointed out, for the chinese, who are the best hope of influencing the NK "government", but so far seem to have no incentive to do so.