Nitrates are a natural byproduct of organic waste (think poop, dead critters, leftover food, that sort of thing) breakdown in a tank. It will naturally work itself out if you don't add to it... but that's a big if.
If you really need to remove it, then I don't think there are any sure-fire ways aside from water changes (assuming your new water doesn't have nitrates in it). A protein skimmer will reduce trates, but I'm not sure if that's the type of option you're looking for.
Things like reducing feedings (both how much you feed and how often you feed) and having a good array of scavengers in your tank will help prevent nitrate buildup in the first place.