Comment by chao-

Depends a lot on the community's topic, purpose, and size.

However, a sign that someone needs to be removed is if you find that the community writ-large treats them as a Missing Stair [0]. If they have refused direct requests from community moderators/leaders to address their behavior, or regressed after a short period of improvement, and if most of the community is required to do work to actively manage their toxicity, that is not sustainable.

It is a mistake I once made in managing a community. There was a person who was able to present as "pleasant enough" when the situation called for it, but who was often toxic when something didn't go perfectly (especially in a 1-on-1 setting). I allowed them to stay for too long, and the community as a whole suffered from the ambient sense of tension and hostility.

[0] https://en.wikipedia.org/wiki/Missing_stair