User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Trust Center > Trusted Catalogs
User
HKCU\software\policies\microsoft\office\16.0\wef\trustedcatalogs
This policy setting allows you to prevent users from using web add-ins. If you enable this policy setting, web add-ins are blocked and all other policy settings in the Trusted Catalogs folder are ignored. If you disable or do not configure this policy setting, apps are allowed. Other policy settings in the Trusted Catalogs folder determine which specific app sources are allowed.