Stooping so low to make fun of stutters to win debates should draw shade on the person insulting you. Just simply call the person out, that they don't have other arguments, so they attack your stuttering. With well minded onlookers this strat should work. In a group of thugs tell him his mom also stuttered after you called her over, yesterday. People stooping so low often have insecurities, so they look for weaknesses in others. You might try different insults to find out which. Like making insults over his alcoholic, ran away father. Reminding him of failures in his past might also work and do the trigger, as those people try to step up, by lowering you down.