I can empathize with you completely. I teach at a university in Taiwan, and this is a common problem. And I agree that university students should be able to sit and listen quietly without the teacher disciplining them.
There is no simple solution for it, but I think it is helpful to consider some of the various alternatives. The easiest is to just ignore them, which might not be a good option if they are too noisy. Another idea is to call on the students who are talking, and challenge them with "excuse me, what did I just say?". A lighter approach is to tease them a little bit. You could say "Hey, your Korean is really improving!" or "Oh, I must be in the wrong classroom. I thought this was English class, not Korean class." You can parrot one of their Korean sentences back to them and ask them "How do I say that in English?" Other ideas: Walk around the class while you are talking and make a point of hovering over the ones that like to talk in Korean. You can also make a note of the "talkers" and from time to time have them come up and write something on the board, such as answers to questions, or some pattern sentences.
I would also recommend trying to understand that students (especially beginners) occasionally do need to communicate in their native language if they don't understand the teacher.
I guess what works best for me is to not make too much a big deal about it, unless they are seriously disrupting the class. And giving the whole class lots of different kinds of language practice (dictations, kinesthetic activities, role plays, etc. ) can do wonders in getting them to speak less Korean and more English.
I hope all this helps.
