Remove co-author for ethical violation?
Personally I think you cannot do it, and I wouldn't even speak in terms of "reasonable" or "unreasonable". If someone makes a contribution to your work, that person is your co-author, deal with it. If a person wants to remove his/her own name, its their private decision, but I can hardly imagine doing that for other contributors without their consent.

When I engage in research with other people, I consider it a sort of binding contract: I do this for you, and I expect you to put my name on the front page. If you don't want to deal with me anymore, it's OK, but next time, not this time.

Furthermore, while I don't want to overthink it and portray pictures of possible dystopian future, I believe such ways of "community shunning" are very dangerous. Maybe today you have a valid reason for it, but you'll open a Pandora's box: first we retroactively remove people for their flaws, then for their views on sensitive topics, then for their thoughts.