You can let them visit. If anyone dies on your lot there will be a gravestone/urn. They won't be added to your household. Even if they live in your household they will be removed from it and you will have a gravestone. In about 3 days the ghost will come out at 11 or midnight.. and if you have a high enough friendship you can invite them to your household ... if you invite them to live with you that's when they get a household slot.
When a non playable ghost visits if the tombstone is on your lot they might break stuff.. so you might want to release the spirit if you never want to see them again, or you might want to put the tombstone in your inventory and carry it to a lot that you can build on (community lot) and make a little cemetery. You can even store them in your inventory basically indefinitely.
