Whenever you sign someone else's Apple ID into your phone, you are turning over all control of your device to someone else. You are now stuck with having to prove the device is actually yours. To do this, you will need your original purchase receipt from Apple or Authorized Apple seller to remove the Activation Lock. A receipt from a third party seller will not be adequate. With this information the Activation Lock can be removed here:
How to remove Activation Lock - Apple Support
You can try to contact Apple directly, but they may just refer you to the previous option to remove the Activation Lock.
Contact Apple for support and service - Apple Support