if you are paid in full with the account the phone was on, just call sprint and tell them to release the phone from your account. thats all you have to do.
if you have a phone to swap with, they will swap the phone or if you do not have a phone they may be able to put a dummy phone on your account if you are keeping the line open.
Moved this to the sprint forum since these steps are the same for any of heir CDMA phones. There is nothing to "unlock" (SIM, etc).