To disable files synchronization in SharePoint Online (SPO) document libraries, you can make use of the document library settings:
Start browsing document library settings page: “Settings –> Library Settings”.
In the document library settings, clic “Advanced settings”.
In the “Advanced settings” page, scroll down until you see the “Offline Client Availability” section and check “No” option.
Just verify the syncronization option is not available in the Document Library.