There's a FEP for a policy setting to say who should be able to reply to a post.
https://socialhub.activitypub.rocks/t/fep-5624-per-object-reply-control-policies/2723
Associated Mastodon issue is at https://github.com/mastodon/mastodon/issues/8565
There's also one for letting the original poster retroactively remove a reply from their post's replies collection, but I misplaced the link.