I have an external monitor hooked up to my laptop. How do I get the laptop monitor to turn off while keeping the desktop monitor on?

Thanks to advice from Fluther, I’m all set with the external monitor. Yay. Now I’m thinking I should turn off the display on the laptop to avoid overheating and to conserve energy. I tried “display options” (from desktop, running Vista) but couldn’t see where to change it. Advice welcome! Thanks in advance.