I am Chetan Savade from Symantec Technical Support team.
It's very easy to achieve please go through the following articles.
1) How to block USB Thumb Drives and USB Hard Drives, but allow specific USB Drives in the Application and Device Control Policy in Symantec Endpoint Protection
2) How to block USB flash drives while allowing other USB devices.
If above articles couldn't help then need to check which SEP features are installed at end machines.
Thanks and Regards,
Technical Support Engineer
End Point Security
Enterprise Technical Support