A lot of people in the comments are missing the point that there's nothing to defederate from yet. Adding "threads.net" to the defederation list is pointless because:
A) Threads isn't using ActiviyPub yet, and we don't know if they ever will.
B) We don't know if that's gonna be the actual domain they would use on ActivityPub.
IMO the whole thing stinks of vaporware. I think Threads is gonna flop and fall into oblivion, but if the future proves me wrong they can certainly block them when that times comes.
Adding threads.net to the defederation list right now only serves to publicly make the statement "We don't like Meta" and serves no technical purpose whatsoever.
I usually reply with "Cool, you won't mind if I install a camera in your bathroom then"