User Configuration > Administrative Templates > Microsoft Office 2016 > Security Settings > Digital Signatures
User
HKCU\software\policies\microsoft\office\16.0\common\signatures
This policy setting allows you to configure Office to only allow certificates from a specific issuer when creating a digital signature. If you enable this policy setting, Office only displays certificates that contain the string you set in the policy. This setting is case-sensitive. For example, a setting of "MyCA" would match an issuer of "MyCA 1"and "MyCA 2", but not "MYCA 3". If you disable or don’t configure this setting, then signing certificates from any issuer can be used.