If you want to disable Landscape mode for your android app ( or a single activity) all you need to do is add,
android:screenOrientation="portrait" to the activity tag in AndroidManifest.xml file.
<activity android:name="YourActivityName" android:icon="@drawable/icon" android:label="Your App Name" android:screenOrientation="portrait">
Now the activity YourActivityName be displayed only in portrait mode.
If you want to do this programatically ie. using Java code. You can do so by adding the below code in the Java class of the activity that you don't want to be displayed in landscape mode.