Want to disable mobile data completely on my phone. How? Thanks.

I'd turned off mobile data in phone setting and mobile data miraculously switched back on when I checked. I stopped using smart phones afterwards. Of course I disabled background data and mobile data usage on all apps with mobile data switched off. Thinking of using smart phone again but only wanted to use voice call and SMS features, what is a sure way to disable mobile data? Go on "my account" at AT&T.com to waste half an hour with Chat_bot and then call AT&T on the phone? They've changed their policy so much these days, would AT&T still have the ability to disable data on a prepaid phone line?