With no real possibility that US sanctions against North Korea will have any real teeth, since the US doesn’t have any trade or diplomatic ties to start with, the struggle to figure out what the US could conceivably do to North Korea has mostly centered on military attacks.
But there are other acts of war possible, and some in the US seem to be advocating a naval blockade of the whole of North Korea, figuring that would be one way to ensure that there’s literally no trade possible.
Blockading an entire country that size would be no msall task, despite the large amounts of US naval assets in the area. That North Korea also has direct links to both China and Russia, their main trading partners, also limits what a blockade could really do, beyond being a declaration of war.
North Korean officials are warning that it would be an act of war, which isn’t really debatable, and are also warning they’d retaliate against any such move, warning it would be a dangerous step toward a nuclear war.
That’s a serious concern irrespective of what act of war the US chooses to escalate the situation in North Korea, as analysts have repeatedly warned that there is no such thing as a “limited” war with North Korea, and anything would quickly escalate into a full-on nuclear exchange.
Trump Administration officials have continued to suggest military means are their preferred choice to anything, except for a total North Korea surrender. Secretary of State Rex Tillerson is the only one who suggest diplomacy as a strategy, and every time he does, he’s publicly rebuked by someone else in the Administration.