We recently told you that there was no way to block the public Microsoft Store but allowing your private corporate Store and while true at the time, is now false. At the recent Microsoft Ignite conference, I tracked down a Store program manager and dug into the problem. He told me this has now been changed so I tested and confirmed the GPO setting works as promised.
You can set the Microsoft Windows 10 Store App to only see the apps you select for your staff with a simple Group Policy Object:
Computer Configuration > Administrative Templates > Windows Components > Store > Only Display The Private Store Windows the Microsoft Store App
Thanks to Jeff at Ignite 2018!