If you add a fermentable sweetener, it will ferment out if you do nothing.
You have a few options depending on if you want it flat or carbonated.
1. Use a non-fermentable sweetener, then you can do whatever you want.
2. Use a fermentable sweetener, and allow it to carb (or not), then pasteurize.
3. Use chemicals to stop the yeast (I've never done this myself, but there are plenty of people here who will tell you how to do it) and sweeten and bottle it flat.
Of course if you're kegging there are other options.