A recent article showed an easy way to stop his rockets. UN declares any NK launch to be hostile, permission granted to shoot them down.

Drones patrol the skies on three sides of the country, tracking any rocket launches. Satellites also scan the country. When a launch is detected, missiles are launched from the drone to hit the rocket in its "Boost Phase" when it is most vulnerable.

Technology is currently available, would need to configure a drone fleet to start the patrols. Nothing NK can do about the drones, they do not have the tech to reach out that far and high.