First off, you need to get the external montior working when it is attached but not mirroring. Then go into System Preferences/Displays and click on the Arrangement tab. In the arrangement screen you will see two small blue screens meant to represtent the two attached screens. Only one of them whill have a white menu bar at the top. Use the mouse to drag the menu bar to the other screen and WoW will not play on that screen.
An alternative would be to close the lid so that only the external screen was active. To do that you need to have the MBP plugged into its AC adapter. You also need an external USB device, like a mouse, to wake up the MBP after the lid is closed. Once the MBP is awoken by USB activity, only the external screen will be in use. Once the external screen has lit up, you can open the lid on the MBP and its screen will remain off.
