Just open regedit and browse to this key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\UsbStor

Notice the value 'Start'

Switch this value to 4, and USB storage devices are disabled.

Switch this value to 3, and USB storage devices are enabled.

For those of you that don't feel like messing around in the registry -

We wrote a program to do it for you