How to solve the North Korean problem once and for all

"Test-launch" an ICBM, landing it on the lawn in front of Government Buildings, PyongYang.

Replace the nuclear payload with flowers and a greeting card "Best wishes to our friends in North Korea, Signed, USA"

Make sure the missile lands more or less intact (but self-destructing any sensitive gear).

There will be no more North Korean missile tests landing in the sea of Japan. Guaranteed.