I think you should stutter on purpose when you start to argue with them. Then tell them with confidence that you stutter. Then they won’t be able to make fun of you because you already told them you stutter. Another thing you can consider is that arguing with that person isn’t worth your time, and save you effort and walk away. This is how you win an argument without having to use any intellect. Lastly, take your time. Don’t have too much pride. If you have too much pride then you will try to respond quickly and and intelligently just to sound smart. Don’t try to sound smart, just be smart! By the way, are you Japanese? I saw your name.