ActivityPub doesn’t just push everything on a server to every federated instance like a fire hose. In the first place, as [email protected] said, it only feeds your content to an instance if somebody on that instance follows you, which you can set to require your manual approval. Your posts could also get pushed if somebody else boosts your post and they have followers on the other instance.
However, if you set an instance block, none of your posts get sent to the instance, period. They would have to resort to scraping. In other words, if you don’t want to give meta your data, just set an instance/domain block.