You can't cancel the data plan, it's a "must have" item with an iPhone purchase.
I think you have to wait 30 days to sell them, meaning you can cancel the iPhone voice/data plan after 30 days and stick your SIM in another cheaper phone.
You just need to call AT&T and tell them that line isn't using an iPhone anymore.
Put the iPhone SIM into a cheaper phone, and read them the IMEI of that.
BAM, no more $30/mo :]