How can I disable the internal display on my MacBook Pro with connected external display?

I used this metod with the MagSafe power cable connected (see http://osxdaily.com/2012/06/15/yet-another-way-to-turn-off-internal-lcd-display- of-macbook-pro-with-lid-open/).

1. Open System Preferences and click “Mission Control”, then click on “Hot Corners”
2. Select a hot corner and pull down the menu to select “Put Display to Sleep”
3. Now connect the external display to the Mac and move the cursor to the newly created sleep corner to turn off the internal display
4. Close the MacBook lid and wait a few seconds before opening the lid again, the internal display should stay off while the external display will be powered on

This worked great under Mac OS X 10.7.4, but after the last update to 10.7.5 it doesn’t work more. The built-in display turns off really, but a few seconds after opening the lid it lights up again. Do you have any suggestions? Dimming the brightness or clamshell mode is not very useful.

MacBook Pro 13-inch,
2,7 GHz Intel Core i7,
Mac OS X Lion 10.7.5 (11G56)

Thanks!