You can mark a post or a group as a draft by setting the
draft frontmatter key to true.
--- postName: Drafts draft: true ---
--- groupName: Writing draft: true ---
Doing so will remove the post (or the group & all its sub-groups and their posts) from everywhere.
Note: In dev mode, files are being read from disk (to avoid reloading and generating index), which means that a draft might be accessible from the address bar.