Exorcising a strong haunting of a family from a legacy of spirits?

I am no expert, but I kept myself, somewhat versed on subjects like this one. This may be a case "group karma" inherited from the grandfather, and his involvement into the "dark arts", and may be something they have to deal with for a period in their life time, or for it's entirety. I only say that as a worst case scenario, because we're dealing with "karma", in which almost all attempts to rid the family of these occurrences are futile, in the event it is something that must be karmically endured to learn a greater spiritual lesson. Until whatever karmic lesson they need to learn is resolved these things may persist.

I suggest that the family change their lifestyle and become devoted and align with a more spiritual one. Prayer is recommended, asking God, or whatever higher power they believe there is, for Divine Assistance, regularly. It may be that, if this is something they must karmically suffer, prayer is the thing to make it tolerable. I should mention, that a credible source on spiritual matters, notes that "dark entities" tend to attack us with greater intensity as we advance in spiritual progression. "There is nothing they'd love more than to bring down anyone of God's beloveds".

It might also be a good idea for the family to get used to these occurrences, and see them as commonplace. They should try not seeing these things as "negative" or "bad", just 'there', and move on about their day. I believe feeding into a certain emotion, brings it about more frequently and with more intensity. If the family project the events as something to "fear", the more like it tends to manifest. Author, Michael Talbot, suggest that psycho paranormal active may be a result of a clairvoyant's psyche "going spastic". Telepathic capabilities are signified by moving and floating objects.

Because there isn't much formal information available on this subject, those who experience them seldom are aware of what they're actually dealing with. There is also a lot of misinformation as well, that can lead to unfortunate outcomes, if not verified by credible sources. Many people grow up with clairvoyant, with little to no understanding of their abilities. Some people get mixed in with the "wrong stuff", and become proficient at using their power, the grandfather being a likely example. Entities from "the other side" offer all sorts of "secret knowledge and power", in return for "spiritual servitude". When met with these beings it is best not to address them directly. I take the stance of, "Want my soul? Take it up with God."

Can you pronounce "Blathering, superstitious, childish bullshit" without spraying spittle?

"Is there any way to remove a very strong haunting and remove a legacy of spirits inherited from a grandfather?"
This depends a bit. Bleeding walls can be fixed by a competent plumber.
The putrid smells are usually decaying rodents under the floor boards.
Spinning shadows are probably a case for the eye doctor.

The boy could find an old book or grandfathers note on how to bind the spirits to the house. Prepare that recipe and find that it works without any further magic. Then a job offer from another city makes them sell the house to the next best villain and move.

"I'm a detective who specializes in supernatural and paranormal cases."
I am Harry Potters forgotten cousin.

yes cast them out by the power of jesus