The South Korean government has been working hard to shut down online gambling sites aimed at Koreans.
181 Korean-language gambling sites have been shut down this year – with the government collaborating with ISPs and portals. Almost 30% of those have been closed in the past month. All these sites are run on servers outside of Korea.
The limitations of blocking the sites is based on language – if they are in Korean then the government will try to block them. However, it realises it's almost impossible to ban non-Korean sites
Article: Crackdown Intensifies on Gambling Web Sites