Any Way To Keep Folders Truly Private?

I think MS has been doing this for awhile, but I want to make sure that any solution works for Windows 10: Any administrator account can muscle into another user's folders. It doesn't matter that the folder isn't shared, or how many times you manually remove the shared access, because the only barrier to entry is a pointless warning prompt. The only thing you can do really AFAIK is manually deny permissions for specific accounts in the security tab of a folder's properties.

But there's nothing stopping an administrator account from making another account that hasn't been blacklisted. And you can't deny the Administrator group unless you want to be locked out of your own folders, since the group denial will trump your user access. Hope I'm missing something here....