I don't see what is the point of preventing foreigners from trading in cryptocurrencies. In today's globalized world, you really cannot impose restrictions. KYC and tracking IPs might help to some extent, but you cannot prevent a South Korean from exchanging cryptocurrencies with foreigners on a decentralized / peer-to-peer exchange. So all this will do is reduce liquidity and hurt traditional Korean exchanges.
There's already been a case of a South Korean fella being prosecuted for acting as a proxy for Chinese traders and this was before this 'ban'.
I'd be very curious to know how many Chinese traders migrated to S Korean exchanges. It seems pointless to me as they can't access banking but maybe they're cashing out P2P. It seems to be accepted that China went there afterwards but I've never seen any evidence for it.