In reality there is not much that can be done. Any military strike by South Korea and its allies would be seen by the North as a provocation to war, which would lead to major destabilization of area as well as a death toll at best not seen since the last Korean war i.e. over a million at worst since the 40’s.
The UN can not apply many more economic sanctions as they are extremely stringent as it is and the Council knows it’ll be the hoi poli who suffer the financial constraints and not the military.
The USA doesn’t want to fight as Iraq & Afghanistan is a major budget drain as well as a loss of life that the population is slowly becoming sick off.
As mentioned already the Chinese don’t want war due to the costs and the huge displacement of hungry North Koreans jumping the border causing a humanitarian crisis. I mean the Chinese can barely feed its own without having to look after millions of Koreans while attempting to have a ding dong with the US.
In reality what’ll happen? Both the North and South will carry on puffing out their chest and eventually the UN will issue the standard English complaint, i.e. a letter of unhappyness asking them not to do it again.
Of course this is my opinion I am sure others will disagree.
P.S. sorry of the long post, it only started off as a sentence and I got carried away.