Lying isn't protected speech.
Yes it is. It's right there within the 1st amendment. Anyone can lie about you and nothing will be done to them, unless you can prove damages in a court of law, which is extremely difficult and requires you to provide evidence that their lies could be used to specifically identify you and that you also suffered measurable damage (i.e. lost money and/or were physically attacked, etc. as a result). And if you're attacked or lost money because someone lied about a group you may have been affiliated with, but they didn't identify you specifically, you get nothing.
Not saying it's right or wrong. Just saying that lying is literally protected speech.
PS: Also, yes, getting someone banned off a "PrIvAtE pLaTfOrM" is censorship. "But...it's PrIvAtE pLaTfOrM" does not make it not censorship. It just means that people like to suck the dicks of corporations like fucking retards.