User Configuration > Administrative Templates > Microsoft Office 2016 > Signing
User
HKCU\software\policies\microsoft\office\16.0\common\signatures
This policy setting controls whether Outlook displays the "Add Signature Services" menu item. If you enable this policy setting, Outlook does not display the "Add Signature Services" menu item on the Signature Line drop-down menu. If you disable or do not configure this policy setting, users can select "Add Signature Services" (from the Signature Line drop-down menu on the Insert tab of the Ribbon in Excel, PowerPoint, and Word) to see a list of signature service providers on Office.com.