How to disable action bar permanently

If you are using Theme.Holo.Light and want to use the Theme.Holo.Light.NoActionBar variant on pre 3.2 devices you can add this to your styles.xml:

<style name="NoActionBar" parent="@android:style/Theme.Holo.Light">
    <item name="android:windowActionBar">false</item>
    <item name="android:windowNoTitle">true</item>
</style> 

and then set it as your activity's theme:

<activity android:theme="@style/NoActionBar" ... />

I use the following code inside my onCreate function:

ActionBar actionBar = getSupportActionBar();
actionBar.hide();

Source: https://developer.android.com/guide/topics/ui/actionbar.html

By setting activity theme in Manifest,

<activity
android:theme="@android:style/Theme.NoTitleBar.Fullscreen"
....
>