As of now we dont have such options its been added to the ideas' list

https://www-secure.symantec.com/connect/idea/automatically-demand-avas-scan-usb-devices-when-plugged

USB will only be scanned when its accessed, will be good if you could post an enhancement requirement for the product.

Does Symantec Endpoint Protection or Symantec Antivirus Scan USB flash drives?

http://service1.symantec.com/SUPPORT/ent-security.nsf/docid/2007100411255848

The File System Auto-Protect scan is the only type which would automatically scan a USB drive connected to the system. This type of scan, by default, would only run when a file is accessed or modified (scans the files when they are written, opened, moved, copied, or run).

Hope this was helpful