You can hide other users but that will create issue when a user reply that message. That user does not know which other users are there. But reply will goes to all users.

In that case you have to set “Separate message”.

what is your message view setup? Threaded or individual?

in “Threaded” view, message recipient is only set in parent message. So it is not possible to reply that message without replying to all recipient.

Suppose user U1 sent a message M1 to user U2 and U3. So M1 recipient list is U1, U2 and U3. So if U2 reply that message as R1, no recipient list is attached to R1. it use M1 recipient list. So if you want to exclude any user from R1 that is not possible as R1 has no recipient list. It uses M1 recipient list.

I do not see any logic to achieve what you want.

In “Individual” view, recipient list also attached to reply messages. In that case you can manually change that.

So you have to set “Separate message” or “Individual” view to achieve what you want.