Take a look at this CodeProject:
Developing a USB Storage Device Protection Tool with C#, it seems to be a good starting point.
To disable USB storage devices it uses a registry key
"HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\usbstor":
void USB_disableAllStorageDevices()
{
RegistryKey key =
Registry.LocalMachine.OpenSubKey
("SYSTEM\\CurrentControlSet\\Services\\UsbStor",true);
if (key != null)
{
key.SetValue("Start", 4, RegistryValueKind.DWord);
}
key.Close();
}