User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Social Connector
User
HKCU\software\policies\microsoft\office\outlook\socialconnector
This policy setting allows you to block the synchronization of contacts between Outlook and the Global Address List (GAL). If you enable this policy setting, GAL contact synchronization is blocked. If you disable or you do not configure this policy setting, GAL contact synchronization is allowed.