The majority of the spirits that abide in our homes are complete harmless. You can spend years living in the home without hearing a single peep from them. Others make themselves known right away as a way of telling you “Hey, I'm here and you're just going to have to live with it.”
Problems arise when the spirits in the home are either malevolent or mischievous to the point of being dangerous. These spirits can attack people in the home and make it miserable to live. You can try and learn who the spirits might be by researching the former occupants of the home and then trying to either help them move on or just learn to live with them.
Unlike other intruders in your home like mice or insects, there aren't many ways to keep the spirits out of the home for good. They are non-corporeal, so you can't just lasso them then throw them out. It's always best to come to a mutual understanding with the spirits and live peaceably. For malevolent and mischievous spirits, you may need to either move from the home or have the house blessed with the hope that it drives the spirits away.
Blessings often alleviate the problems for a while, but eventually the spirits are able to return and the harassment starts anew.