The units you are talking about at walmart are probably deionizers, some of them have built in carbon scrubbers. They work. If you have an outdoor vent, ozone is the #1 odor fighter, but it's not really safe to use in confined spaces, so is really ideal for removing the odor from outdoor vents, not the interior spaces. I keep hearing a lot of people saying ONA is a lifesaver. Whatever you choose, realize your closet is going to have an odor. If you need to kill ventilation odor, go with ozone or carbon (ozone if you can afford it). If you need the interior odor reduced I would get any combination of deionizer, carbon filter, ONA, and run it in the room the closet is attached too.