Help with Group Policy to deny removable media access (except certain users)

[Gazb]

As the title says: Group Policy to deny removable media access (except certain users)

I appear to be having a couple of issues, applying a group policy that denies access to removable media on all domain computers, with the except that administrators are able to access removable media.

• 
  I have a policy that is being applied to the Domain Computers OU, that denies access to removable media.

User Configuration-->Policies-->Administrative Templates-->System-->Removable Storage Access

(I've also tried: Computer Configuration-->Policies-->Administrative Templates-->System-->Removable Storage Access)

• 
  I then have a policy that applies to the Users OU, to allow access to removable media, however in the Scope I have set the filter to a specific Security Group which I will add people who need this access to.

I am unable to get the allow access policy (user OU) to override the deny access policy (domain computers OU), when I set the deny access via:
Computer Configuration-->Policies-->Administrative Templates-->System-->Removable Storage Access

When I set it in: User Configuration-->Policies-->Administrative Templates-->System-->Removable Storage Access
It allows all users access.

I know I'm probably missing something obvious and simple, so any advice would be great.
(I have also tried using the Loopback processing mode, when setting the deny under computer configuration but that didn't seem to fix it either)