How do I prepare a new device in Configurator without updating the device iOS?
Place the required version iOS ipsw file here:

~/Library/Containers/com.apple.configurator/Data/Library/Caches/com.apple.configurator/Firmware/

Use the Edit Custom iOS Versions command
In Apple Configurator, hold down the Option key while choosing Devices > Edit Custom iOS Versions. You can then add or remove custom versions of iOS software, such as developer previews.

source: http://support.apple.com/kb/HT200241?viewlocale=en_US