External Signature Services Menu for Office must be suppressed.

Users can select Add Signature Services (from the Signature Line drop-down menu on the Insert tab of the Ribbon in Excel 2010, PowerPoint 2010, and Word 2010) to see a list of signature service providers on the Microsoft Office Web site. If your organization has policies that govern the use of external resources such as signature providers or Office Marketplace, allowing users to access the Add Signature Services menu item might enable them to violate those policies.