There is a member in my group who has taken an intense dislike to another member. Both members are respectful in their postings, and neither member violates the terms of membership.

However, the dislike is so strong that one member has left groups.io because he can't stand seeing posts from the other member.

To be able to keep both members in the group, there needs to be an option for the first member to mute the second member (and thus, never see the second member's posts). It would probably be easiest to implement by making the muting two-way. That is, the second member won't ever see the first member's posts, either, and thus won't be in a position of trying to reply to the first member when the reply will never be seen by the first member.