How should I handle when people mock me or make fun of me at work?

How should I handle when people mock me or make fun of me at work? I don't stutter much, I think it's more of a stammer when I can't get certain letters/words out. For example, I'll ask a question and on a certain word, i'll get stuck and make sounds and my mouth will be moving all weird...Then the person I'm talking to will mock me. It's very hurtful and I just usually walk away not saying anything.