As another tip discusses, you can prevent users from installing any new devices (including USB drives) via Group Policy. However, if you only want to block write access to USB drives, you can add this registry entry. Users could plug in drives to access and copy files to the computer but can’t copy or move files onto the drive.
Create the following key and DWORD value:
Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Control\StorageDevicePolicies
DWORD Name: WriteProtect
DWORD Value: 1
