When you need to block external access to your most sensitive documents, Office 365 Data Loss Prevention policies and sensitivity labels combine to find and protect the documents. A really simple policy is enough to detect and block external access, and is covered by Office 365 E3 licenses. If you have E5 licenses, you can consider auto-label policies to find and protect sensitive documents at scale.
https://office365itpros.com/2020/07/06/data-loss-prevention-with-sensitivity-labels/
