We know what ships go in and out of their ports. It's not very many. We can very easily set up a weapons embargo and make sure that no ballistic missiles come out. We can also threaten sanctions on anyone who cares to do business with them. The next 'Yemeni' ship we stop, when we have the right sanctions in place, will be seized and impounded, with the U.N.'s blessing.
North Korea is going to set new records for unpopularity. Unable to sell WMD or ballistic missiles, they'll be isolated and helpless.
We will see the escalation of actions on both sides. N. Korea will incrementally step up its threat to produce nukes from previously deactivated reactors. U.S. may well have U.N. declare N. Korea an international outlaw. It will slap economic sanctions in incremental steps, while enlisting China and Russia's help.
This will buy time to finish Iraq. After that, who knows what will happen ? The regime change of N. Korea from within is the best way to solve this crisis.