See -
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.
http://www.symantec.com/business/support/index?page=content&id=TECH106304&locale=en_US
How to block USB flash drives while allowing other USB devices.
http://www.symantec.com/business/support/index?page=content&id=TECH104299&actp=search&viewlocale=en_US&searchid=1288975028344
For your idea of the user requesting a password, you can submit the idea for possible inclusion in a future release of SEP.
https://www-secure.symantec.com/connect/node/add/idea?add_context=691