If you are on a feature phone plan, and you put your sim card into a smart phone, AT&T will detect it and automatically sign you up for a data plan (you'll get a text when that happens.)

It used to be, if that happened, that you could switch your sim card back into the feature phone, call up AT&T, and they would remove the data plan.

If you still have an old feature phone you could try putting the new sim card into it and calling AT&T.  But since (I assume) they gave you a big discount on the smart phone, they may not allow you to drop the data plan.