In order to truly disable IPv6, you must disable it in the registry in the following KB929852 article:

How to disable IP version 6 (IPv6) or its specific components in Windows 7, in Windows Vista, in Windows Server 2008 R2, and in Windows Server 2008  

But, you can use Jeff Guillet's (Exchange MVP)   ADMX and ADML files.

Download File:
IPv6Configuration.zip  

This ZIP file contains both the ADMX and ADML files:

• IPv6Configuration.admx - Copy this file to %SYSTEMROOT%\PolicyDefinitions
• IPv6Configuration.adml - Copy this file to %SYSTEMROOT&\PolicyDefinitions\en-US (Replace en-US with your country's language, as necessary)

Now use the Group Policy Management Console (GPMC) to configure the IPv6 settings.



For example, right click on Default Domain Policy and click Edit.



The new policy will be located under Computer Configuration > Policies > Administrative Templates > Network > IPv6 Configuration.



Read Explain tab.



Here, you can configure the following IPv6 settings: 

• Enable all IPv6 components (Windows default)
• Disable all IPv6 components (the setting you probably want)
• Disable 6to4
• Disable ISATAP
• Disable Teredo
• Disable Teredo and 6to4
• Disable all tunnel interfaces
• Disable all LAN and PPP interfaces
• Disable all LAN, PPP and tunnel interfaces
• Prefer IPv4 over IPv6

Testing with Domain Controller 2008 and Windows 7 Client.

Edit Default Domain Policy, go to Computer Configuration > Policies > Administrative Templates > Network > IPv6 Configuration, Open IPv6 Configuration Policy and set on Disable all IPv6 componentsand click OK.



Run GPUPDATE /FORCE.



Now, run IPCONFIG /ALL on Windows 7 before Restarting.



Now, Restart Windows 7 and run IPCONFIG /All again.




Additional Info:

Navigate to the following registry key: 

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\DisabledComponents(32-bit DWORD value)


Configuration combination                        DisabledComponents value
Disable all tunnel interfaces                              0x01
Disable 6to4                                               0x02
Disable ISATAP                                             0x04
Disable Teredo                                             0x08
Disable all LAN and PPP interfaces                         0x10
Disable all LAN, PPP, and tunnel interfaces                0x11
Prefer IPv4 over IPv6                                      0x20
Disable Teredo and 6to4                                    0xA
Disable IPv6 over all interfaces and prefer IPv4 to IPv6   0xFF


Important:

Before disable IPv6 on Windows Server 2008 or 2008 R2 or SBS 2008 and ..., please read this article:

Should I Disable IPv6? No... (Ace Fekay - MVP)