How to cancel my grandma's 3g plan on her iPhone for AT&T?

Well my grandma came home complaining about me using her iPhone and using her 3G. She said AT&T has detected that she is using a smartphone, and now they added 3G to her plan for $20 - $30 per month.AT&T doesn't have the right to do that right? I mean she never uses her 3G plan but she is charged $20 - $30 on something that she doesn't even use. I have only used her iPhone to download a game from the app store once and they charged $27 for that. I asked if she could take the sim card out of her iPhone and put it in one of her old phones but she won't do it she is as stubborn as a mule and all she does is call on it. So can anyone help me cancel this AT&T 3G on her phone? I mean she doesn't use her iPhone for anything other than calling.

Call at&T. Do you expect the messageboard to call them for you?