Clear boundaries. I have only two hard rules that would make me act immediately: no player-versus-player behaviour and no sexual violence. I repeat these two rules to each new player. I state them in a direct, no chance to misunderstand, manner.

I had something similar. It went like: No r?pe, no necrophilia, no needless child murdering, or other overly edgy shit. There is no debating this. I will immediately call you out and start a fight over that.

Never forbid Player vs Player, tho. As long as both sides were ok with that they could bash each others heads in. They constantly beat each other or kindled their clothes on. But miraculously never killed each other.