Here are som useful links:
http://support.microsoft.com/kb/555324
http://diaryproducts.net/about/operating_systems/windows/disable_usb_sticks
http://www.intelliadmin.com/blog/2006/06/disable-usb-drives-remotely.html
Shortly:
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesUsbStorStart to 4 to disable
HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesUsbStorStart to 3 to enable
