If you have contract with Cingular, and you pay them for data and voice, and it is blackberry from Cingular- play dumb with their customer support.
Call Cingular customer support and tell them you want to delete your
BIS account. If they ask for PIN and IMEI you have them.
If they ask why- tell them you are getting too much spam and you want to create new account with new email address.
They can do it for you. I don't see why not.