How to disable Sync Button for SharePoint Document libraries
By default, OneDrive for Business is used to sync the documents of a SharePoint document library down to local computer for offline access. Any user that has access to the document library has the option to Sync one or more libraries. In some cases company rights and policies request to disable this option for all users. If you log into your SharePoint site, and click on a document library, you should see a Sync button on the top right corner.
If you want to hide that button, or just disable in global settings, you need to click the gear in the top right and go to Site Settings. Choose link that says Search and offline availability.
You will see a line that says Offline Client Availability, and set the the option to no, then hit OK