How do you tell them? Pick a day when you want to start. As soon as someone speaks in Korean, say, "Shhh!" Make a stern face at them and then say, "English!" Everyone should catch on after a couple of warnings. Then, wave a board marker in front of the class and say, "If you speak in Korean, you'll get a black nose!" If they don't understand that, say, "Korean, black nose!" Make motions of coloring your own nose. They should get it. It's up to you if you want to use a warning system before coloring their noses or if you want to just immediately blacken their noses. Board markers are safe to use because they easily wipe or wash off. Of course, you shouldn't allow them to wash their faces until class is over or they'll take advantage of this new Go to the Bathroom Free card you just gave them.
How strict you are on this No Korean thing is up to you. I get my students even if they slip up and say one word in Korean. Some students think it's the most terrible thing you can do. Use your own judgement about them. With some, I let them color their own noses so they have a measure of control. Some are very dramatic with pleading and begging and promising never to speak in Korean again. If they can make a good plea for themselves in English, I usually give them a second chance.
After a nose is black and the student is still jabbering away in Korean, start drawing whiskers. Eventually, you may have a class of cats.
Of course, you can use that Purple Kiss threat, but it's nice to add variety to your day and the kids look cute as cats.
