I will assume your machines are capable of having the ADM templates applied. With that said, read this support KB from Microsoft and ensure you have verified you have things setup as suggested.
Recommendations for managing Group Policy administrative template (.adm) files - ADM template article
What your trying to accomplish is not easy to manage, audit, or verify that it has applied successfully. Disabling USBs is one thing, but tracking whose plugging in the devices is quite another.
Even with Windows 7 and newer group policy in Windows 2008 R2 this task is better left to third party tools. Microsoft does not have any interface to easily control and manage this option.


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