Unsigned add-ins in PowerPoint must be blocked with no Trust Bar Notification to the user.

This policy setting controls whether the specified Office application notifies users when unsigned application add-ins are loaded or silently disable such add-ins without notification. This policy setting only applies if the "Require that application add-ins are signed by Trusted Publisher" policy setting is enabled, which prevents users from changing this policy setting. If this policy setting is enabled, applications automatically disable unsigned add-ins without informing users. If this policy setting is disabled, if this application is configured to require that all add-ins be signed by a trusted publisher, any unsigned add-ins the application loads will be disabled and the application will display the Trust Bar at the top of the active window. The Trust Bar contains a message that informs users about the unsigned add-in. If this policy setting is not configured, the disable behavior applies, and in addition, users can configure this requirement themselves in the "Add-ins" category of the Trust Center for the application.