Whether or not he can keep the phone is a question of whether or not it was a gift or a loan. If he is not going to give it back voluntarily, the only way to get it will be a small claims law suit for the value of the phone. The service account is a different matter. If the account is in your name you have every right to change or cancel the account. In fact, it is possible that by making the changes he has made to the account is has committed a crime - wire fraud, identity theft, or something similar depending on the specific details. You should be able to go to the carrier and get the account terminated with no problem.