GPO to force the flash drive to use certain drive letters

I work at a school. I am having trouble off and on with patrons who use their thumb drives. What happens is that the thumb drive gets assigned a drive letter which is in use by a network drive so it doesn't get visible.

What I need is a way either through a GPO or local setting to force the flash drive to use certain drive letters or NOT use certain letters.
The USBDLM service is not my favourite option ;)