My personal opinion is to take it at face value. You now know they’re not good people and can base your expectations around that behaviour. Don’t take it personally because we know that people will continue to behave this way throughout life in regards to our stutter… we can’t control it. if it continues to be a problem from the same people I would politely explain to them that you do have a speech impediment and that you don’t appreciate them mocking your disability. If it continues to be an issue I’d present it to HR. You can’t control what people do or say but what you can do is inform your bosses and HR that you’re being harassed about it so that way people at least take you seriously.