I have a hard time dealing with this myself. On one hand, I would want to be comfortable enough with myself to be able to take a joke about stuttering and just laugh it off. On the other hand, it is something which is a daily struggle for me in my life, and is more often than not more antagonizing psychologically than physically. In your case I would have reacted in the same way as you, since I don't really think they way they acted was okay. Hopefully it was just a one time thing. People are jerks sometimes. Stay strong.