Revoking a DocGen license doesn't stop a Viewer-license user from creating and running merge actions — the merge buttons remain on the Salesforce page and can't be disabled natively. Today the only workaround is Salesforce Conditional Visibility Rules to hide the buttons. Honor the license tier natively: Viewer-license users without a DocGen license should be blocked from creating and running Document/Email Merge actions, without requiring a manual page-layout workaround.