Hi,
Ideally USB flash drives are blocked for security reason.
If you have enabled it please review it's necessity.
Check risk logs on clients machines, check whether it' new threat ? or happening due to system vulnerability ?
To check whether it's new threat or not ?
Run SEP support tool & submit files to Symantec for analysis.
http://www.symantec.com/techsupp/home_homeoffice/products/sep/Sep_SupportTool.exe
System vulnerability can be avoided with following ways.
Use all three features i.e Antivirus/Antispyware , Proactive Threat Protection , Network Threat Protection.
Make sure all three feature have latest definitions.
Upgrade OS with latest service pack and windows updates.
Upgrade third party software's with latest patches. (e.g acrobat reader)
Disable auto-run .
Use latest Symantec Antivirus version i.e RU7 MP1 or 12.1 RU1
As you said you are able to remove it manually ? which threat it is ? Symantec is detecting with manual scan or how ?