Cool, so pick an instance that plans to defederate with them and you're golden.
Personally, I think all the anti-Threads stuff is paranoid rhetoric and I'd rather see how it pans out. My instance admin agrees so we'll see how it goes.
Point is you can choose because that's the entire point of the fediverse. And it's why I don't understand why folks are expending so much energy writing paranoid pieces on this topic when they could just defederate and move on.
No, that's literally how this works.
If you don't like an ActivityPub participant you block it. It's in the architecture.
And given the current fediverse is already a tiny fraction of total social media activity, if a bunch of anti-Threads instances hive off to form their own fediverse subgroup, it'll basically be a no-op from their perspective. They'll just keep talking to each other off in a little corner by themselves just like they're doing today. That's kinda the whole damn point of a federated architecture.