That just sucks. I've only ever had somebody do something like that to me one time, and I will never forget how shitty that made me feel. But I think it's important to realize that people who are that blatantly rude are few and far between. Most people are understanding when they realize you stutter, at least in my experience. If it is somebody you know, maybe you can let them know how hurtful that is and hopefully they will understand.