Fediverse

The only official way is for the admin of your instance to defederate from the instance you want blocked, but that affects every registered user on your instance, even if they're against the action.

I just released a user script that lets you block instances yourself on the client-side as a regular user - basically just removes post and comments from the HTML if they match your block list.

Yes. A lot of that is going on with instances being blocked en masse for allowing too many spam/bot accounts without any corresponding high quality activity coming from users registered with that instance. It's very much in flux right now, with instance administrators trying to figure out which metrics to use and what lists to trust, but I imagine a more mature/robust process will be used by most serious instances soon enough.

Let them join…and be ignored.

I see the threat in the sheer developing power of these giants, making all the shiny tools people were wanting, making their service too attractive to be ignored.