No offense, but this is out of your control. How are you going to banish something from a huge building without access to every room? If what you're describing is true, that is a fully manifested spirit, and a very fricken strong one at that. If you try something and fail, you may be the next one going to the ER.
The best thing you can do for the family is ward their apartment(s) from this spirit being capable of entering. Not the building but the individual apartments/rooms. Then give each one of the family members an amulet you've charged that can protect them when they leave their home until they move.
I'm sorry, but in order to successfully get rid of this spirit, that I would say is something far more than a lower spirit, you would need to have access to the entire building, cleanse and banish in every room, and you would need help, at least one or two more people experienced in magick, if not an entire coven. Afterward, everyone involved would need to protect themselves because those type of spirits don't let things go, it may follow someone, even you.
Bottom line, please don't try to get rid of this spirit. More harm than good will come from it. Just use protections. Since it is not your property or this family's (they don't own it) you have very little choice.