My advice would be to reply simply, firmly, and sincerely "Please do not do that ever again. I have a stutter, and I can't help it". This, or close to it, has been my line for a long time. It usually results in a profuse apology. When you make it clear how hurtful, and not funny it is, people respond appropriately. And if they don't, you've got good info on who in your life is a shit person.