Disable "Make Available Offline"
Computer Configuration\Administrative Templates\Network\Offline Files
Prevents users from making network files and folders available offline.
This policy removes the Make Available Offline option from the File menu and from all context menus in Windows Explorer. As a result, users cannot designate files to be saved on their computer for offline use.
However, this policy does not prevent the system from saving local copies of files that reside on network shares designated for automatic caching.
This policy appears in the Computer Configuration and User Configuration folders. If both policies are configured, the setting in Computer Configuration takes precedence over the setting in User Configuration.