You aren't able to. AT&T requires all Smartphones on postpaid plans to have a data plan. If you add a smartphone to your plan and don't add an appropriate data plan, AT&T will modify your current data plan or add a data plan to your line. It's within the Terms of Service you agree to when you sign up for service.
GoPhone customers have some options in regards to data plans. The $25 and $50 plans don't include data -- although you can buy data packs for the $25 plan. The $40 and $60 plans have a set amount of data with the ability to buy additional data.
*The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.