Partially. It’d help a little bit. But if you federate with another instance that doesn’t block it, that data will still get out.
Essentially the protocol would have to be updated to carry a blacklist that all instances would adhere to, but basically via an honor system.
The only method that could truly protect your data would be whitelisting, but that would severely hamper and fracture the fediverse.
Looking into it, aren’t both of these only Mastodon and not part of ActivityPub itself? I can’t find details on them outside of Mastodon.
And what prevents the post from getting published to other instances from different sources?