Activation lock prevents the activation of iPhone's unless the previous owner has removed the device from his account. This is to prevent stolen iPhones from being used. Since you own the phone legally (presumably, so did the person you bought it from), you should be able to contact the previous owner. Since they have forgotten their password, they will need to go through the steps to recover it, then remove the iPhone from their account. Until that is done, you are out of luck.