Windows – Group Policy: Block access to \\localhost\C$

We have a restricted Windows 7 computer that hides and prevents non-admin users from accessing the C Drive using the following policies:

• Prevent access to drives from My Computer
• Hide these specified drives in My Computer

However, they are able to circumvent this by typing the following into Explorer:
\\localhost\C$

How can I disable this path but allow other UNC paths. For example they are allowed to access a shared folder on a different computer. eg. \\192.168.2.1\SharedTransfer

Note: Simply Enabling the Group Policy: Remove Run menu from Start Menu will not work as this blocks all UNC paths.

Update
The user, called Site User, is not a member of the Administrator group directly or indirectly. It is only a member of the group Users.

Accessed the following from:
Control Panel\All Control Panel Items\Administrative Tools\Computer Management (Local) > System Tools > Local Users and Groups

Local Users and Groups
 - Groups
      - Administators
             - Administrator
             - Service User (our admin user)
      - Users
             - NT AUTHORITY\Auntenticated Users (S-1-5-11)
             - NT AUTHORITY\INTERACTIVE (S-1-5-4)
             - Site User (user account in question)