Easy - tell them "I don't want to have conversations with you about the other women. I want a chance to know each one of you as an individual, and I'm not going to listen to rumor, gossip, and badmouthing each other. I'm not going to settle your grievances with each other, if there's someone in the house you don't like, don't talk to her."
of course, that would never work because the producers WANT the women to talk smack about each other, and if there are two that don't like each other they WILL be on a two-on-one date together. But Peter has the ability to walk away from a conversation that is just one woman talking about another being fake, or lying, or god help us, "not here for the right reasons."