How to disable the Recent Tasks/Apps button

This will close the RecentActivity dialog. Put it in your activity class.

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);

        if (!hasFocus) {
            windowCloseHandler.postDelayed(windowCloserRunnable, 250);
        }
    }

    private void toggleRecents() {
        Intent closeRecents = new Intent("com.android.systemui.recent.action.TOGGLE_RECENTS");
        closeRecents.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_EXCLUDE_FROM_RECENTS);
        ComponentName recents = new ComponentName("com.android.systemui", "com.android.systemui.recent.RecentsActivity");
        closeRecents.setComponent(recents);
        this.startActivity(closeRecents);
    }

    private Handler windowCloseHandler = new Handler();
    private Runnable windowCloserRunnable = new Runnable() {
        @Override
        public void run() {
            ActivityManager am = (ActivityManager)getApplicationContext().getSystemService(Context.ACTIVITY_SERVICE);
            ComponentName cn = am.getRunningTasks(1).get(0).topActivity;

            if (cn != null && cn.getClassName().equals("com.android.systemui.recent.RecentsActivity")) {
                toggleRecents();
            }
        }
    }

You will need to put the following permission in your manifest.

    <uses-permission android:name="android.permission.GET_TASKS" />

Have you tried this code?

public void onWindowFocusChanged(boolean hasFocus) {
    super.onWindowFocusChanged(hasFocus);


    Log.d("Focus debug", "Focus changed !");

if(!hasFocus) {
    Log.d("Focus debug", "Lost focus !");

    Intent closeDialog = new Intent(Intent.ACTION_CLOSE_SYSTEM_DIALOGS);
    sendBroadcast(closeDialog);
}
}

From: http://www.juliencavandoli.com/how-to-disable-recent-apps-dialog-on-long-press-home-button/