I think, in this case, the best we can do is provided a "city on a hill" example for them to aspire to, and hope eventually they cast off their dictatorship on their own. And we should keep our eyes open for reformers within North Korea, who might pop up, in the hopes we can act in ways that strengthen their hands, and weaken those of their opponents.
That sounds weak, especially from the perspective of a country like ours that has grown accustomed to seeing every foreign problem as something calling for a military solution. But history shows that what seem like permanently-entrenched dictatorships can often fall rather suddenly, to the surprise of everyone, without any significant foreign military intervention. The obvious example is the whole Soviet bloc, which as of the mid-to-late 80s looked like it wouldn't be going anywhere any time soon, and then suddenly the Berlin Wall is down, and the communist regimes start toppling one after another. Nobody had to invade East Germany to start that process. It just turned out that the dictatorship there was a lot weaker than the CIA or anyone else thought.
There are other examples, too. Mobutu Sese Seko, dictator of Zaire, was around for decades, ruling with an iron fist, then suddenly was gone, with no US interventio. Marcos, Suhuorto, Franco, and so on. Sometimes the dictator dies of natural causes and the people see that as chance for a change and sometimes he's taken out from within, or flees. I'm not saying North Korea is necessarily ripe for that, but I think it's important to realize we don't have a good track record of knowing when countries are ripe for it. It often takes us by surprise. So, we needn't think of it as a choice between military action and leaving people under a cruel dictatorship forever. Sometimes the best thing the doctor can do is give the patient time to recover on his own.