Banning the exchange of digital assets, in my opinion, is like banning the internet. It's not feasible in the long run. Would also be a disappointing move to see coming from a democratic country that has some degree of property rights.
It's more like banning a specific set of companies from opening legal websites in Korea. It's entirely doable, and has been done many times (e.g., porn). I won't comment on its morality: case by case, I guess.
The ban's purpose is not to stop the most determined, persistent crypto-anarchists from realizing their dream. Its purpose is to scare away your average retired dads and office workers from pouring their life savings into BTC, because otherwise the country will have to feed their children when the bubble pops.
(Unlike crypto-anarchists, most Korean people and the government believe that it's the country's duty to keep children well fed and educated, even if their parents are raging idiots. Whether we actually follow through is... debatable, I guess.)