If you want to block users to access other users root folders you need to remove some permissions on your root drive like the image describes.

Please beware that after doing this operation some windows updates will fail to install. Whenever you want to install some windows updates that fail to install add Users group back on your root c: drive with  Read & Execute, List and Read permissions.

After you install those windows updates you can remove again the Users group from your root c: drive but only after reboot if a server reboot is requested by Windows.