Lemmy is based on a pull model, so if nobody on a different instance subscribes then it doesn't show up on anybody else's feeds. If an admin doesn't want that in their "All" feeds, they can block the instance.
Just make sure it's on its own instance with nothing else, something like that is bound to be EXTREMELY noisy, and not all admins are gonna be happy about it. I assume that's what you meant by closed?