Completely erase sprint from iPhone 4s

Well i got this sprint iPhone 4S that my friend sold me but it seems that his account is still active on this device (has no sim card in but still has sprint signal bars) I would tell him about it but hes in Amsterdam and hes coming back in 3 months. I don't feel like waiting 3 months just so he could cancel his sprint account on my iPhone. Is there any way to completely delete all that sprint stuff from the iPhone? I tried restoring it but it still activates. I already got the iPhone jailbroken with ifile on it. can anyone tell me what to do?