1. What does it mean?
철벽 literally means an iron wall. 철벽을 치다 describes making it clear that a romantic approach is not welcome or leaving little room for someone to read interest into an interaction.
Also written or said as cheolbyeok
2. The nuance
It is not a challenge to push harder. A person can be friendly while maintaining a boundary, and describing them as 철벽 does not mean their refusal is secretly encouragement.
3. Examples
“연애 생각 없다고 분명히 말했더니 친구가 철벽이라고 하더라.”
I clearly said I wasn't interested in dating, and my friend called it an iron wall.
“상대가 철벽을 치면 더 설득하려 하지 말고 존중해야지.”
If someone sets a firm boundary, respect it instead of trying harder to persuade them.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.