Really depends on how it is said to me. If they seem like they are doing it in a "friendly" (like you can tell they were thinking it maybe a joke) I usualy will just politely respond "No, I just have a stutter". I try not to sound angry or upset because its not fair to make someone feel shitty over an honest mistake when they are trying to be friendly. I can understand that they may not have ever heard someone stutter before. Hell I didn't meet another person who had a stutter until my final year in college. If you are concerned about being vindictive just watch your tone and wording since that can make all the difference. However there are times where I will look them dead in the eyes and say "I have a stuttering problem". There are certain places and times where no matter how a person speaks you should not interrupt them or make "silly jokes".