and then cut off the greater Fediverse as a whole once their user base is established and much larger than ours.

I have bad news for you, then. Or good news, or whatever. Threads already has 160 million users. I thought one of the talking points was that Threads was going to overwhelm the Fediverse when it connected?

We don’t want them interacting with us at all.

Don't speak for everyone. The whole point of the Fediverse is that everyone can have different opinions and nobody can unilaterally cut someone else out of it.

If you don't want to interact with Threads, there are plenty of instances that have already defederated and lots of clients allow users to block by instance. You don't have to. But if someone else does want to engage with Threads users, you shouldn't try to stop that.