How to deactivate an iPhone 4?

Okay so i bought an iphone off someone from craigslist and they didnt deactivate it so their number is still activated and i have the phone so obviously i cant call them to deactivate it… Theres no sim card in the iphone 4 so im really confused please help

Its a sprint or verizon phone. You cant deactivate it without the person on the account authorization. And if they owe money they will not let them do it. So you will have a nice paperweight for ur desk:)