How do you permanently disable the laptop screen?

I've setup my laptop on my TV via HDMI and have XBMC installed as a media center. The laptop is closed next to my entertainment unit, and I control it using a wireless keyboard. I noticed that on the odd occasion when I open the lid, I see the same stuff on the laptop screen that I see on my TV screen... meaning XBMC is rendering both screens at once.

Now, my laptop isn't the most powerful in the world, and I like squeezing every bit of juice out of it that I can... Seeing that the display is being rendered on 2 different displays, is this taking up additional system resources that could be better used elsewhere?

I'd really like to know if there's any way to disable the laptop screen altogether, and only use the HDMI port.

I know this can be done directly in the Ubuntu display properties, where you can chose to turn a particular display on or off. However, it still seems to be active when running solely in an XBMC session.