Hello dtmgallagher,
Unfortunately, there is no way to set the USB device to read-only due primarily to the fact that each of our test environments require write access in order to function correctly. However, if ever believe that the USB device has been compromised, you have the option of restoring it with the Diagnostics CD (or DVD in the case of Service Center 9.5 & up).
Please let me know if you have any other questions or issues.
Thank you,
Kris