The only possible punishment to North Korea, 조선민주주의인민공화국, for its nuclear test seems to be hit and run.
Military action is out. The risks in human lives is far too high - especially considering Seoul's (서울) close location to the Northern border.
Economic sanctions are also out. They would isolate the North even more, and it would victimize the population much harder than the decision taking leaders.
One possibility is a sudden and hefty sanction. During one month, there will be no contact with North Korea. No food, no fuel, no contacts at any level. After that, go back to full economic exchanges to help the poor fellows out.
A one month sanction would certainly be felt in the North. The government would have big difficulties explaining to their population what was going on, when suddenly electricity and heating got cut off suddenly. And it would be difficult to explain to workers in foreign factories, why they were locked out from their jobs for one month without pay.
It would send the signal "the rest of the world will and can act".