You might have entities inside of you. I went to a shaman and she found two entities and sent them packing. They were not past lives or alternate personalities. One was a bundle of negative, hateful energy from my mother, my brother, and my ex-boyfriend. The other was a robot-like creature that I had created in my own mind for protection. It was time to let him go, too. It wasn't cheap and it took hours, but it was well worth it.
interesting story, even though potentially not very fun for you!
I have done things like muscle testing or dowsing in the past and gotten clearly wrong answers, but I would still go with it, if it makes sense to you on some level. It does not cost anything to initially just try yourself to heal it (if it is real). Just go into meditation, call upon your ultimate good divine healers and helpers, and ask them to free you from John, cut any potential cords, and fix any potentially energetic damage he might have wrought. It might take more than one session, and if by the third one he is still there, maybe consider getting outside help... (like Visitor suggested). If there is any potential lessons to be learned ask your helpers to enlighten you. Depending on what kind of spirit John is, getting answers from him, why he is there - could be confused, or misleading - so take potential answers you get form him with a pinch of salt. After all, unless you specifically invited Spirit to come and live with you, he is there uninvited.
Love and Light!