You did the right thing OP. If anything, I would have demanded to see the manager and make them repeat exactly what they said. I’m a very kind person in general, but I absolutely will not stand someone insulting me or anyone else for something that we cannot help. There are far too many people that lack the empathy to try to understand how difficult stuttering can be on a person, and I have no issue making this fact known to them. Be proud of yourself, I know from experience that having a stutter makes it all the more difficult to stand up for oneself, and we often get stepped on by other people. You’re a better person than they’ll ever be, and I pity those who have to deal with their ignorance and malice on a daily basis.