If it is impossible to take away their trump card in a surprise assault, then we are only left with the option of finding some way for them to compromise with us and give up their trump card. After their trump card has been so successful it is not within reason to think that they would surrender it in another agreement (as good as the last one?).
If things are as you opine, then we are essentially out of options other than buying them off (how much in taxes would we like to pay to buy off NK blackmail?)
The deal is this. After decades of near (military) parity with the South, the North is in free fall. Their factories can churn out all the spare T-55, and T-62 parts they want, but on the whole, the KPA is showing its age, and has no way to modernize. The ROKs, on the other hand, are lean, powerful, and high tech. Every year that goes by, the invasion option loses some of its menace.
Enter the bomb. It is only useful against the U.S. and Japan (Kim would be the most hated Korean in all of their history, and for all time to come, for destroying Seoul. In a culture that is obsessive about 'losing face', that is not in the cards) To deliver this bomb to us, Kim needs missiles.
Enter the ABM defense system (especially the air to air laser platform the USAF is working on, for boost phase intercept). See where this is going? Once we rob Kim of his ability to deliver, he'll be left high and dry.
The NK problem is unique, we'll just need to think outside the box to get these particular bastards.