Realistically, we can't nuke the little shits, short of general world war. China, Taiwan, South Korea, and Japan are all to some degree in the possible fallout patterns, which makes nuclear weapons bigger than the little-bitty artillery ones a nonstarter, and even those would be unacceptable to all those players as a first-strike weapon.
But we could still cut the head off that snake, and a lot of its neck, with a massive conventional strike. Doesn't mean we can prevent their shelling of Seoul in the process, though.