As others have mentioned, the only sure-fire way is to call AT&T and tell them to disabled "Pay per use data". Actually, it's a good idea to do this even WITH a data plan, so you are immune to "glitches" that occassionally happen where your data plan doesn't get applied properly and they charge you per kilobyte (and instead the data either works under the plan, or not at all). It's rare, but it does happen (happened to my boss once).