I had some kids like that once. I had to get strict with them, but I didn't raise my voice. Instead we do this little snap and point. It really works!! After that, look at them square in the eye and say, "No", and if that doesn't work threaten to get their parents in there and they will quiet down after that. Even better tell them at the beginning of class that if their good today they will get a special treat. Our girls like to do the song "Bear Hunt", and if they are good, we'll play it for them.