When you install Office 2016, your users gain access to the Microsoft Store (no reason not to try and sell when you can, right, am I right, am I right ?)
You may not want to have your user base downloading and installing applications onto your internal estate. So, how do you disable the behaviour ?
Microsoft have provided a GPO to turn off the store / remove the button but it only works in Outlook !
This GPO setting is available in the User Configuration section at Microsoft Outlook 2016 –> Outlook Options –> Other. Enable the policy “Hide the Office Store button”

Like I say, it’s under “Outlook Options” so only affects Outlook. How to disable for other Office Applications ?
The article at https://docs.microsoft.com/en-us/previous-versions/office/office-2013-resource-kit/cc178992(v=office.15) does prove a reg key (for 2013) that will block access to the stores and this will likely work for 2016 (with an appropriate adjustment to the path of the key for 2016 to access). However, there is a simpler way.
Just use GPOs to drive the “Trust Center” to not allow Store Apps to start. This setting is at Microsoft Outlook 2016 –> Security Settings –> Trust Center –> Trusted Catalogs. IN there is a value “Block Office Store” – simply enable that.

Now, when users access the store, rather than getting this 👇

Users will see this 👇

i.e. the Store button is still visible but they cannot download or use apps from the Microsoft Store but you, as an administrator, can still release applications you do want them to use to them.