When you create a new post it is first saved on your instance (where your user is registered). Then your post will be pushed to all other instances (servers) where any user has registered the magazine (sub) you are posting to. The instances will then save the post to their database.
If you want a post to be deleted you have to contact the administrator of the original instance (as you said). once the post is deleted there, a delete message should be pushed to all other instances that received the post. The instances will then also delete the post to their database.