I will start with the last one, it is probably the easiest. According to Buddhism there is no permanent realm where you stay forever so even though a person is a ghost that doesn't mean that they are permanently stuck there.
Why people become ghosts? According to the teachings there is a realm called the realm of the Hungry Ghosts, and it is taught that beings go there because they had a lot of craving. I heard one teacher say that people get reborn in ghost realms because of attachment. He talked about being careful when we die not to be attached to the things that we have left behind because it may keep us hovering around and not willing to go on to our next life, to what we could go on to.
The story goes that there was a very devout elderly Thai man who wanted to donate a monk's hut. He was very happy, the builders worked very hard and finished the hut. Then he wanted to donate a bathroom so work was started on the bathroom but before it was finished he got very ill and he died. Now, the teacher at that particular Wat had psychic powers, so he went into meditation, to see where this devout Thai man had been reborn.
What the teacher saw was that this devout Thai man was a ghost hanging around the bathroom because it wasn't finished! The teacher was very concerned that even though this person had done a lot of good actions he was stuck, hanging around the bathroom because he worried about the bathroom not being finished. So the teacher came out of meditation and told the relatives. When they heard this they finished the bathroom very quickly. The teacher then went back into meditation and saw the ghost of the man was sitting smiling at the bathroom, being happy that it was finished. So he actually talked to the ghost and said, "Think of your good Kamma and go on, will you?" So the person started to do this and then had a nice big smile and then went on to a nicer realm.
Whether it is true or not, the moral of the story is, "Try not to be attached to our work". If things are unfinished at the end of the day, okay, just reflect that you may die that night and try not to be so attached to things, which is a good lesson for us because how many of us get attached to our work or to things not finished? Just to understand there is always going to be more dirty laundry left. Human work is never finished. So it is important to let things go if you don't want to get caught in the ghost realm.
As far as attachment to people, that's more difficult. So it's important to learn how to wish people well and understand that we can't end other people's suffering, or be with them to help them all throughout time. To wish them well and from what I know it seems that people meet again in next lives, so rather than getting attached to the person and that particular body we will say, "Oh, well, maybe next life, maybe I will recognize you again". Steve and I seemed to have recognized each other this life, so who knows.
And there is the story about the Buddha, when he was still Prince Siddhartha recognizing Yasodhara. So spending lifetimes working on the Paramis together appears to be important and trying not to be too attached to the manifestation and the form of a person in this life. This may help us not to become a ghost.
Now, if they can influence people who are alive? I am not too sure about that although there are stories in Thailand. One was about a monk who was practising and a ghost was hitting him on the head. He was getting a headache and wondering why. Whether they are true or not I don't know. But there are also stories that if a person has a lot of good Kamma and developed a lot of Compassion/Lovingkindness towards these beings in these realms, then they won't harm them.
Also if one reflects on the Buddha, the power of this good reflection can help, as it showed in my dream, the spirits disappeared, I do believe it helped. Apparently the ghosts don't want to harm people that have these pure thoughts in their mind. And there is the teaching of the Buddha about the benefits of Lovingkindness, that spirits won't harm you because if we think about them and how they may be suffering in that realm, we develop Compassion/Lovingkindness to them and they won't harm us.
In fact, one of the nuns at Wat Kow Tahm felt that she had seen a ghost. American monk Emanuel Sherman died at Wat Kow Tahm many years ago. So one day this nun comes to me and says, "Sherman is hanging around here. We need the foreigners to give him good wishes and to tell him, 'Go on'. He won't listen to the Thai." So whether it is true or not, I felt it wouldn't hurt to do a chanting night with the Assistants for him, wishing him well and encourage him to go on. So what's we got together for this purpose and we hope, if he was hanging around, that it helped him.
So I hope I don't get too attached to Wat Kow Tahm that I hang around. I try to remember that. Maybe she was trying to give me a teaching, I don't know. A warning.