Laugh it off. Don’t be offended. Laugh with them. Make fun of yourself before they do. Lighten up. These co workers are not laughing at your character or your behavior, it’s just a silly thing. My old speech therapist used to suggest this to me. You have to de-sensitize yourself to making mistakes. Take be fear and shame out of it, that’s your enemy and will force you to do all kinds of abnormal speech behavior and avoidance which you don’t want. When I stutter I try and say “blah, I can’t talk today”, something like that, and re-start. I am farrr from perfect but it’s a good technique.