This is something that took me a long time to learn. I used to make a lot of excuses for people who teased me about stuttering because I was insecure about it myself. Nowadays I do my best to call people out for making fun of me by saying, "Hey, I've got a stutter," or "That's not funny, dude." Nothing too crazy, but just enough to check the person and remind them that it's really not okay to tease about it.