North Korea is a dangerous rogue state and the United State is right to impose sanctions. North Korea would just use trade to promote its weapons programs and further oppress its people. The best way to deal with states like North Korea is to deny them resources, and sanctions achieve this goal.
Sanctions are not always the best way to achieve a desired result. In the case of North Korea though, sanctions seem to be working. Unfortunately, the most noticeable impact is the general population of North Korea starving. There is very little money coming into the country. The only reason the people of North Korea haven't overthrown their government yet, is because the rulers control the media, and try to indoctrinate them into how to think. The pressure on them is starting to show through their ever more desperate and threatening actions. All it will take is a bad harvest year. North Korea already has terrible land for farming and feeding itself. If enough people get hungry enough, they will overthrow the government and find out what the rest of the world is really like.