I agree with hosts, this needs to be resolved. I get mostly South Koreans stay at our guest house and their excuse is that they don't speak English well enough to understand the House Rules. Most will not even take a look at our Descriptions, they just book. So, when we smelled the smoke from the guest house bathroom after the guest left, and mind you I charged them for extra cleaning, etc.. Sometimes its not enough.
I made sure that when another South Korean guests book or thinks of booking, my first response after the greeting is, "Please make sure to read our guest house description carefully. This is a NO - Smoking guest house." Then, I was able to get some Korean friends to get us "No - Smoking" signs in Korean, and we've put them in the guest house.
This helped a lot. But, doesn't resolve the issue. More needs to be done.