Why on earth did you get a Tilt if you ABSOLUTELY, POSITIVELY won't use data? Thats like getting a Viper for grocery trips. You're weird, dude.
Anyway, I would first suggest reading the printed manual. If that doesnt have it (which I'd expect, because most manuals are useless these days) I would contact tech support at AT&T via email. They usually get back to me in a day and their advice actually works.
Dont be suprised if you cant. Actually turning off one of the radios in a cell phone is usually not an option. Enabling Airplane Mode is a common feature, but that disables everything. There isnt much practical reason to disable just one mode. Also, you wont be saving yourself from accidentally using data. You can still get data on the 1x radio or whatever they call it, just much slower.
I think you will just have to practice a little discipline and not use data features. Which on that thing means accessing just the text messages and thats about it.
Thats a very expensive texter you got there. Please tell me you have plans to use data someday.
EDIT:
Phonescoop says that the battery lasts longer in GSM-only mode, but I'm not sure what that means.
http://www.phonescoop.com/phones/phone.php?p=1297
It could just mean that if you dont have 3G access and are only able to use 1x then it will last longer, which is true. But I have not yet seen a phone that lets you choose to only use one or the other.