I would call them out. Just say “what the hell do you find so funny? My stuttering?” They will feel like ass clowns and you will look brave for not only admitting you stutter but for also standing up to fools that still think stuttering is funny. Acknowledging and owning your stuttering is really the only way forward but it does feel really good once you do it. It’s about addressing the fear directly. Either way sorry this happened to you. I can definitely relate. Hope you can go easy on yourself and still celebrate the achievement that is graduating. Congratulations!