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.