It may appear as I am being the devil's advocate, but a military intervention which would alleviate this problem successfully (i.e., Overthrowing the regime) would almost certainly result in some sort of nuclear strike.
North Korea's arsenal is capable of reaching almost of all South Korea and some parts of Japan. If their Taepodong-2 missiles are operational, it puts much more of the world at risk. Furthermore, due to the success of North Korean social engineering, any direct assassination of the regime would result in a massive conventional war - an inevitable event in any military intervention.
North Korea currently has the world's largest standing army, as well as the world's largest submarine force. They are capable of calling up almost 75% of their population up for military service at any one time. North Korea also maintains a stockpile of chemical weapons, and they have been known to experiment with weaponizing biological systems. Furthermore, they have a special forces division numbering around 120,000 men, which is believed to be trained to Western standards. If true, this means North Korea also has the world's largest special operations force. (Source:
http://www.defense.gov/News/NewsArticle.aspx?ID=16791)
Presentation of North Korean missile ranges:

Once again, the main point is that any attempt to remove the regime will result in a nuclear strike - they have nothing to lose.