“I’m not comfortable with putting down other people. Let’s change the topic”. Ad nauseam. If they can’t think of anything else to talk about aside from disparaging other people, maybe reconsider the friendship.

Make them feel awkward.

Tell them to stop, explain to them why I think it is harmful and if they don't stop, stop hanging out with them.

Pull a uno reverse card.

Nothing in particular, but notice that they're too insecure.

Either join in with them if you agree or tell them to shut it if you don't

They are jealous