How can I block GPP Drive Mapping in a child OU?

For certain accounts, I want to block the GPP drive mappings that we set based on security groups.

I've tried creating a User-scoped GPO directly on that OU and disabling the drive mappings but that doesn't seem to discard the mappings applied at a higher level OU.  

<?xml version="1.0" encoding="UTF-8"?><Drives disabled="1" clsid="{8FDDCC1A-0C3C-43cd-A6B4-71A6DF20DA8C}"/>