Do they know that you stutter as a condition? Perhaps a chat individually or redirecting a question at them such as "what is the point in mocking me for a speech impediment that is difficult to control and that I have lived with for all but my entire life?" I have never been actually mocked by someone, and generally apologize for my stutter if it seems to have an impediment or to explain why I am acting in a certain way. Generally people are less likely to crack a joke if they see you as dealing with something. If it gets bad beyond this, a chat with someone higher up may be necessary.