Abusive And Harmful Ghost

I'd like to begin by saying I have never personally seen a ghost or anything paranormal, though I definitely believe in their existence. This story is not mine, it's my fiancé's story (who we'll call Roger), but we were wondering if we could possibly find advice about what to do on here. So here goes.

It wasn't long after we started dating that he told me he sees ghosts. Ever since he was little, he would have "playmates" that only he could see and communicate with. Eventually he would help them with whatever they needed for their souls to rest in peace. Growing up, he thought of it as more like a game, though now he considers it a responsibility of his to help them. At the moment he's trying to help a younger ghost, she's only ten years old, who we will call Sarah. She stays in his room at night and almost watches out for him, because he's helping her. She does occasional pranks such as moving books around his room or hiding necessary items under his bed. But she's never hurt him in any way. The first thing that disturbed me about all this was that on occasion, he'll allow Sarah to possess him. It's frightening, especially when I'm around, as he is unconscious though she speaks through him. She talks to me that way sometimes, and from what I can tell, she means no harm through it. However, she's not the only ghost he talks to.

There's another ghost whose name we'll say is John. For the longest time, Roger told me that John was his ex-boyfriend who died in a car accident. It wasn't until we did a little research that we found John never existed. None of Roger's friends or family remember him or the accident, and Sarah claims that some nights there's a weird energy in my his room, which is why she keeps watch for him. It wasn't until last fall that we actually had problems with John. He began to appear to Roger and threaten him physically and sexually on a regular basis. He had an extreme hatred for me, since for a year he had manipulated Roger into a physical relationship, which I had just replaced. A friend of mine gave me advice that ghosts cannot physically harm a person, which I wasn't sure if it was true or not, but I trusted it for the time being and Roger tried to get rid of John. The threats got worse however, and Roger's family told me he spent much time alone in his room and rarely slept at night anymore. And that was when the actual problems started. Multiple times John possessed Roger either to taunt me or to harm him. He even went as far as to break Roger's fingers when I told him to leave us alone. He oftentimes would drain Roger's energy and sometimes paralyze his body if Roger said the wrong things. Finally one morning, John tried to kill Roger and nearly succeeded, except I started praying as hard as I could for his life to be spared (I'm a strong Christian and Roger's Catholic) and all at once, John disappeared.

It was almost two months until he came back. When he did come back, he wasn't as menacing as he first was. He seemed slightly subdued, though he still threatened that he could take Roger's life. He said the only thing that would stop him, would be (I know this sounds strange) for me and Roger to have sex. I'm against that sort of thing for religious reasons as we're not yet married, but we decided we'd wait for two years, and we told John that in two years' time, he could kill Roger if we hadn't done as he asked. So he swore to leave us be until then. Sarah, however, claims that she can talk to him quite often and that he has been staying around the house. She talks to him, and says he's not a very bad person, and seems almost nice, but that there's a menacing aura around him (which I feel at times when I'm in Roger's room). She also says that there are such things as demons who survive off sexual energy alone, and that could possibly be what John is.

Just the past few weeks, John's been back again, threatening and abusing Roger sometimes even three times a week. We're at the end of ideas, we can't have the house blessed because then Sarah would be sent away as well as John.

Does anyone have any experience with these types of things? How is it that you can get rid of them for once and for all? Is it truly possible that he could kill Roger if he wanted to?

Thanks.