There were programs to train special forces in South Korea to go into Pyongyang and assassinate Kim Il-sung (Jong-Il's father) back in '70s, but it came to nothing due to thawing of relationship. Personally I suspect certain units in South Korean special forces have been training under such a mission scenario for years.
As to why South doesn't try to unite North by force, a few reasons. Firstly, such a reunification after the resumed civil war (remember, they are technically at war with one another), or even Germany-styled absorption of one half by the other will cripple the economy of South as well as North. German economy suffered for years due to the ramifications of unification, and East Germany was in no way as bad a shape as North Korea is.
Secondly, South Korea's capital-and the nation's capital before the division-Seoul is only 50km away from the border, and the metropolitan area including the satellite city is home to almost 20 million people. North Korea has thousands of field artillery pointed across the borders. In effect, half the population of South Korea-not to mention all the economic, cultural and military assets-are hostages to the North. And now with the development of nuclear weapon by Kim's regime in North, the military victory becomes less and less likely. Even if they can prevail due to superior military equipments across the board, such a pyrrhic victory will benefit no one in Korea.
There are all sorts of other reasons military action with quick victory, or pre-emptive strike in any case, is out, such as a provision in South Korean constitution that directs the government to strive for peaceful unification (how exactly it is implemented is a different matter), need for cooperation with US military and government, and sizeable pro-North elements within the South. As much as it is frustrating for everyone having to listen to threats of a madman, the best course of action would be to keep engaging in various forms of dialogue (with appropriate use of stick and carrot), and to be alert so that the South (and USA) can retaliate with potent and full military might should all hope for peaceful resolution be dashed by all-out assault from the North.