Actually, Korea is not that bad in terms of DCC implementation.

In all restaurants I've been to in Seoul, I've never seen DCC. All transactions are done in KRW, and you sign on the pad.

In department stores (like Lotte Department, not Duty Free), you sign on the pad as well, but before that, you have the choice of currency on the pad, so not hard to control at all.

In Duty Free, the price is denominated in USD, so no DCC to worry about.

I've never been to a place where the DCC is locked, but maybe I've been away from obvious tourist traps. Mostly I go to restaurants packed with locals, not ones catering foreigners.