In the short run, you can’t. Every plausible approach has been tried by the last several administrations, and all of them have failed.
Here are your options:
1. A preemptive war is not a viable approach because even if you somehow managed to destroy all of the North Korean nuclear weapons before they could be used, the North Korean army has enough men and artillery to level Seoul. South Korea will understandably never agree to this.
2. More sanctions won’t work because the country is already isolated, and the Chinese aren’t willing to push any harder.
3. Engagement has been tried by the South Koreans on numerous occasions. It always fails, because the North Korean government (correctly) views it as a threat to their regime.
4. Leaning on China is the current favorite solution. It is true that the Chinese leadership has no use for the Korean regime, but it is equally true that the Chinese view the status quo as the best of a menu of bad options, most of which would ultimately result in an American ally with troops on its border. The North Korean government knows this, and is therefore prepared to ignore Chinese attempts to rein it in. Don’t expect much help from this quarter.
Two additional observations:
1. The best ultimate solution to North Korea would be a military coup inspired and controlled by the Chinese. The North Korean regime is aware of this, which is why anyone close to the throne who is perceived to be too close to China is likely to end up being executed in some particularly gruesome way.
2. The actions of the North Korean government are perfectly rational if you begin with the premise that it has no interest in the welfare of its people; it only cares about its own survival. Occasional flurries of public militarism help keep the people in line.
The bottom line is that, for the foreseeable future, all we can do is remain vigilant without overreacting and hope that the Chinese can help at least a little bit.