A couple of main options:
If you want to remove someone from mentions, just remove their account address from the reply.
If someone else wants to stop getting notifications about a conversation, they can mute the thread by clicking โฏ and then "Mute conversation". This will only affect posts made to that particular thread. More info here:
โก๏ธ https://fedi.tips/muting-conversation-threads-in-mastodon/