Countries can decide not to trade or interact with a given country which is their business. There is no obligation to do business with NK. And deciding to not do business with a country is not the same thing as an act of war. You are not saying that our actions have resulted in NK being able to say we have declared war on NK (which we haven't). Surely that's not your point.
An analogy could be the DOS, denial of service, action on the internet. Interrupting trade is an act of war, just as DOS is a crime. Recall the Arab embargo of oil to the West was an act of war. Also, a Declaration of War is an act by Congress in this country, although other countries may have their own interpretation. In the case of N Kor, as far as they are concerned, whatever they say is a declaration of war is a declaration of war.