I'm making an app for jailbreak that lock the device when the user launch the app. I've tried GSEventLockDevice(); from GraphicsServices.framework but this does not work properly because it locks the screen but does not power off the screen. Is there another way to lock the screen without MobileSubstrate?
Just as another alternative, check out this answer, which uses SBDimScreen(). You could use that in conjunction with GSEventLockDevice().
It appears that you may now (iOS 5+) need to add an entitlement to your app to use this call successfully. If you haven't done that before, here is an example of how to do so. Obviously, in this case, the entitlement in question must be changed to com.apple.backboard.client.
You also might see if @VictorRonin has experimented with this, as he commented on the question I linked to. I tested SBDimScreen() on a jailbroken iOS 4.2.1 device, but my newer devices are currently unavailable to run this test. I'll try it later on iOS 5.x and post an update.
Note: the answer I linked to from Elias has a different call altogether for iOS 6.