BB:Presenting NK with an ultimatum could eliminate its nukes short of war, but then that wouldn't actually be negotiation--so I'm all for it. But for such an ultimatum to work, it would need to be credible. So we would need actually to be willing to fight and win such a war. If you don't think it's possible to do this--and therefore possible to convey that it's possible--no ultimatum will work.
We can fight and win such a war, at horrible cost. If our ultimatum creates the impression that we're ignoring that cost, then it creates the impression that we are bluffing. My ultimatum recognizes that the loss of Seoul would be horrible, and tries to avoid war if possible on that grounds. I did everything I could to eliminate the North Korean nuclear program through Plan A, diplomacy. I concede everything but the essential issue, the North's ability to sell nukes to al-Qaeda. If that fails, then Plan B is war and the loss of Seoul. No one will take your threat of war seriously if it is accompanied by fairy tales about eliminating the North's artillery in a preemptive strike