I also came up with welcome2atlantis's "stun'im"-idea myself, but I like WhimsiCat's "let him die and revive him later, as much as it takes him to understand"-idea more, as Stephanie else would look like picking a fight. Also because it is nicely sadistic. But if she would draw huge aggro for that....maybe she can revive him, once a tank can aggro directly afterwards, but that would throw of the attack-schedule of the party.
Stephanie has a big audience right now. It is less about showing, that MC's strategy works for other groups, but more about how BS attackers aren't the hottest shit and healers and tanks deserve due. Actually MC can thank God, that such a bastard found his way into Steff's party and such a great opportunity arrived that early. He would make a wonderful prime example to put other a-hole attackers into their place and prove MC's point. So Stephanie's goals must be to come up with a strategy, which emphasizes on, how the a-holes actions jeopardize the group's victory and therefore their spend money, and after everyone sees, that this would lead into the groups demise without a doubt, she has to turn the situation around again (with the help of her teammates excluding the attacker from hell)/rightfully punished that attacker.
In the end harvest's idea is the best. Just keep him on a short leash. Just heal him and buff him enough for him to not die by AoE-dmg, but don't give him any other buffs, so he can't pull enough aggro until he either snaps at her really bad (kick him out), gets himself killed or apologizes.