How to inactivate a sprint iPhone?

I recently bought a phone from someone on Craigslist, Jennifer. She's a student and only had the phone for a couple days & I trust her. I bought the phone as a present, and just now gave the present. When they went to activate the phone under their own Sprint account, they were told that the phone has not been disconnected under Jennifer's name. I was able to make calls using Jennifer's phone and under her number. I'm assuming the ESN is good, or else it wouldnt have been able to call my cellphone. Seeing as i can't call her and don't have other contact information for her, how do I get around this? Can i disconnect the phone myself?

Personally I would go to a sprint store and tell them the situation. If you have your saved emails from CL and Jennifer, bring them to sprint as well. Sprint can take care of all that in the store. They can contact her for you.

She has to do it but it sounds like you are going to have a big problem.

If she only purchased and activated it a few days ago, she can cancel her contract with Sprint but she will have to return the phone if she does.

Obviously she is under contract so in order for her to cancel her contract without returning the phone she will have to pay an early termination fee of $350.00 in order for you to activate it.

Take it to Sprint so that they can contact her to see what your options are.