Legally:
Iphone 3g requires a $30 iphone data plan. If you are no longer using the iphone 3g, you are not required to continue to carry that data plan.
iPhone first generation also requies $20 iphone data/text (200 SMS) or $30 (data+1500 SMS) or $40 unlimited data/text.
Illegally:
Just call ATT up and tell them you are no longer using the iphone 3g. Give them the IMEI of a reqgular phone to update their systems. Also tell them to block all internet data from the account. That way you won't face extra data charges if your wife accidently tries to use data.
You cannot remove the iphone data plans via OLAM. Only CS can remove them.