Depends on the setting. If it’s at work or school, ignore them and if it gets out of hand report them. If it’s outside work or school, get in their face, ask them to repeat that and never back down, they won’t do that again. Worse comes to worse, you’ll get your ass kicked. Had my fair share of my ass being kicked for standing up for myself as a kid when people made fun of my stutter. Just stay safe, I’m not encouraging violence or putting yourself in danger, just know how to pick your fights.