In the Manifest, you can set the screenOrientation to landscape. It would look something like this in the XML:
<activity android:name="MyActivity"
android:screenOrientation="landscape"
android:configChanges="keyboardHidden|orientation|screenSize">
...
</activity>
Where MyActivity is the one you want to stay in landscape.
The android:configChanges=... line prevents onResume(), onPause() from being called when the screen is rotated. Without this line, the rotation will stay as you requested but the calls will still be made.
Note: keyboardHidden and orientation are required for < Android 3.2 (API level 13), and all three options are required 3.2 or above, not just orientation.