What happened when they tried to build a nuke (even if the test was faked)? We gave them a few billion dollars a year to not do it again. We've given them every incentive to develop long range missiles that we can pay them a few billion more to not use.
You options for taking military action against them are either a massive preemptive nuclear strike that would have to hit civilian areas with a military presence, or writing off at least a quarter of South Korea's civilian population and all military forces within a hundred miles of the demilitarized zone. NK has spent years digging in bunkers and artillery positions right up to the DMZ, and their contingency plan if attacked involves something like 10 million artillery shells into South Korea in the 3 hours (that comes out to about 925 shells per
second.) Seoul has 10 million people and would be reduced to rubble in seconds. This is as close to mutually assured destruction as you can get without nukes (which I don't believe NK has.) The only thing we can do is wait for the government to crumble from within.
I was recommended this documentary: "Children of the Secret State," about North Korea's refugees inside the country and the conditions most of the 22 million people there live in. They eat dirt, or grass if they're lucky, and cannibalism is common as people get more desperate. It's upsetting to see what's happening and know there's no way to help.