Should groups of people that wish to establish a society where what the-one-law defines as harm is endorsed in their law be left to do so if the only option is to use force to stop them?
As long as the "groups of people" Harm no one in doing so, they should be allowed. Given a locale where some such people already live, we can achieve this in a very peacefully.
The parent post here was based upon a misinterpretation of the sentence it replies to. The original post describes a group which justifies Harm inflicted internally. Not good...
They harm each other, but harm no one in setting it up, the initial generation is voluntary, their descendants will be in a society that actively harms without a possibility of escape.
Perhaps I tragically misread your question. That sounds like a real problem seen previously in communes set up in the USA back in the 1970s. I will reply again to the original query.