Dude, just shame them as soon as they laugh at you. “I have a stutter so..” usually does the trick. People like to have fun but they don’t want to feel like their actual assholes for making fun of an impediment. Also helps for you to be lighthearted about it. Most people don’t know stutterers or aren’t aware of the stutterers they know. So it’s an honest mistake most of the time, not a deliberate attempt to put you down. But if you react negatively it mostly makes people not take you seriously. Hope this helps.