Meta is going to do embrace, extend and extinguish on ActivityPub (https://en.wikipedia.org/wiki/Embrace,_extend,_and_extinguish) as they did with XMPP.
It would be nice if you implemented a way to easily block them into Mastodon admin interface, but it's also evident you won't, last but not least because of what you've written in the first post of this thread, and the fact you mentioned @mosseri in it.
So we need, in spite of you, to share knowledge on how to do that other ways on any already ActivityPub federated platform (Mastodon, Pleroma, Pixelfed, PeerTube and so on).
CC: @vantablack@FediPact --- @cwebber@evan (the latter 2 contributed defining the ActivityPub protocol)
NatureMC