You can perform an exorcism or blessing. You local preist may help you with the latter and i once assisted a priest with one. You can seal the room with light if you know how to projecet your conscousness or use protection spells
Just think rationally and it's gone. Then you can go find the true explanation for whatever phenomenon you mistook for a ghost.
They're not real.
An astral presence left over after a sescised person.