What is the most precise way to prevent external users from accessing sensitive documents within a SharePoint Online document library, even if they have a sharing link?

The most precise way to prevent external users from accessing sensitive documents within a SharePoint Online document library, even if they possess a sharing link, is to revoke their access directly or implement stricter access controls. First, you can specifically remove the external user's permissions from the document or library. This can be done by going to 'Manage Access' for the document or library and removing the individual external user's entry. This immediately cuts off their access, regardless of any sharing links they may have. Secondly, if the sharing link was created with specific permissions (e.g., 'Anyone with the link' or 'Specific people'), you can either delete the sharing link itself, which will immediately invalidate it, or modify the link to remove external access. You can also change the link type to 'People in your organization' to restrict access to internal users only. The most robust solution involves a combination of these approaches. For example, if you are unsure who might have received a sharing link, removing the external user's direct permissions from the document library and deleting all 'Anyone with the link' sharing links ensures no external access remains. Finally, sensitivity labels configured with encryption and access restrictions, applied to the documents, can override existing sharing permissions, providing an additional layer of security. This ensures that even if a link exists, the external user cannot decrypt the document without the necessary permissions.