I had this problem as well as I wanted to constrain my game to only landscape mode. I put this in my OnLaunched handler for App.xaml:
Windows.Graphics.Display.DisplayProperties.AutoRotationPreferences=Windows.Graphics.Display.DisplayOrientations.Landscape;However I noted that in the simulator it seemed to ignore this whereas on the hardware tablet I tested on it seemed to behave appropriately. The AutoRotationPreferences are bit flags so you can or together all the orientations you want to allow.
from : http://*.com/questions/12614508/how-to-limit-orientation-in-metro-apps