> Aren't we talking about blocking and muting known user handles?
This is the same as blocking known spam domains and IP blocks in email. They just switch to a new one. Which is best dealt with at the remote end since the server has a better chance of being able to identify new accounts which are related to blocked ones - Instagram even suggests this for you when you block someone: "...and any future accounts they may create". Having to do this in A's client is much more difficult because it lacks the information that would be useful.
> What happens if B does the same before A had a chance to block them?
This is the same as blocking known spam domains and IP blocks in email. They just switch to a new one. Which is best dealt with at the remote end since the server has a better chance of being able to identify new accounts which are related to blocked ones - Instagram even suggests this for you when you block someone: "...and any future accounts they may create". Having to do this in A's client is much more difficult because it lacks the information that would be useful.
> What happens if B does the same before A had a chance to block them?
A fair point.