The thing is friends always mock or be nasty to their fellow friends for no reason at all but merely because they want a good laugh. You may notice they might have done the same for another friend and really didn't mean to offend or hurt in anyway, but in your case they might have forgotten that stuttering meets the definition of a disability and so should be immuned to criticism, mockery or insult. Did they really have a reason to be hostile to you? If not then they just wanted to have a laugh is all. You do not need to educate them on why they shouldn't make fun of a disability if you trust that they won't do the same unto a stranger. I would advise you to just forgive them this mistake and forget it. When this happens to me my instant reaction is to just stare for at least ten second in the distance - giving them no clue as to what I feel. If you laugh with them too, then this won't make them think. They wouldn't be the right state of mind to do some serious thinking so trying to talk them out of it would make you stutter more or they would try to defend it as being a joke. Having a cold distant reaction makes them think and it works for me. But you mustn't hold them negatively in your mind - because that wouldn't do you any good. Just forget it thereafter.