Under these circumstances, you may ned to hire a law firm that has a lawyer fluent in Korean and with an office in Korea, to assert the same rights.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Why on earth would an internet service provider in Korea give a hoot about your United States-based Digital Millennium Copyright Act take down notice? While cyberspace has extraordinarily porous borders, nation states still exist and are defined, in large part, by their own bodies of law. There are no worldwide copyright rights and no worldwide mechanism to enforce whatever rights each particular nation state chooses to recognize.
In short, the copyrights that you're trying to enforce exist ONLY in the United States unless some other nation chooses to recognize them. Many do, some don't.
South Korea does -- but only to the extent it must under the copyright treaties that it has ratified. Like US law, South Korean copyright law immunizes internet services providers from liability for infringements by third parties who publish infringing content. Their law also goes the next step of creating a DMCA-like take down mechanism -- but you have to use THEIR mechanism, not ours.
If you want to enforce your US-created copyrights in South Korea then you need to hire an attorney in South Korea (or one in the US who's authorized to practice there). See http://www.buyusa.gov/korea/en/lawfirmcontacts.html