I would recommend that first, you set some basic ground rules for your class regarding the need to speak politely in class to both you and the other students.

If the students respond with the comments you mentioned in your post, I would probe them and ask "Why do you feel we don't need to talk about this, or why do you feel that is a stupid comment?

Remember that you goal as a teacher is to help students develop their listening, speaking reading and writing skills.