I doubt you'll like the answer, but you should ban those folks. And if people complain about them getting banned, ban them too.
I've come to see that, unlike many liberals, you actually do believe what you say and have an honest commitment to creating an open platform, which is why I say that I don't think you'll like the answer. But it's better in the long term. This site has undergone repeated purges. Quite prominent users, even mods, have been banned. While it's painful, it contributes to making a platform where the users you actually do want come.