the fact that you let it go the first time made him do it again and again which is completely your fault and you should not give him the benefit of the doubt at your own expense no matter who is that person. this has never happened to me in adulthood, but if it did, I would have immediately confronted them and escalated the situation even if it ended up in a physical confrontation dealing with stuttering alone everyday is a nightmare, but being laughed at in a professional setting for something that is out of your control is just absurd. if you are not a confrontational person then I recommend recording these incidents, because trust me, no one can tolerate such childish acts forever and you are going to need the evidence at some point