Go to settings, then application manager, then slide it over to ALL applications. Scroll down til you find S voice then click on it. It will give you two options "force close" or "disable" click disable.
Now it won't open anymore when you double click the menu button, and it won't be running in the background.
Just remember after you disable it, it won't be listed in your application manager>all applications alphabetically anymore, it will be all the way down at the very bottom of the list. I disabled mine and went to enable it again and was worried because I couldn't find it anywhere, it was just all the way at the bottom. Same goes for any other apps you disable. You can disable S voice and other apps that are running in the background all the time. But only a select few like All share, S suggest, Color, yahoo news and yahoo finance.
Those are the ones I've disabled and it's sped my phone up quite a bit, and yahoo news isn't eating my data anymore.
Hope this helps!