First, fraud. Many, if not most, cases of poltergeists are really fraudulent. Let us ignore those.
Assuming a "legitimate" case, parapsychology generally does not think a spirit is involved, but rather some form of emotional "effect", typically caused by a teenager. In this situation, the teenager is currently living there or in the vicinity. However, the actual physical cause of the effect is not explained, therefore disregarding it as being caused by a spirit seems to me to merely be because parapsychology just wants desperately to be considered a "real" science. So -- it is possible that a spirit would be responding to the emotional effects of the teenager in such a case.
Now assume a "legitimate" case, where there is no living person who might be the cause. Then in this case, a spirit is involved. (Again, assuming this is a non-fraudulent case, and is not due to other normal physical world manifestations, like bad wiring causing lights to flicker as the house shifts in the wind or expands/contracts due to air pressure, and so forth.)
There is no need for the spirit in such a case to be a ghost, however, and plenty of reason for it not to be.
Now I make a simple assertion - I cannot prove this statement: There are many categories and kinds spirit beings, and many of them are hostile to humans.
So - the poltergeist may be a non-ghost spirit. Since ghosts in general have zip-nada-nothing in the way of causing physical manifestation, dramatic physical effects may in general be attributed to the non-ghost spirit category. I say "in general" due to the fact that ghosts generally cannot even move a paperclip. But I am not a ghost expert so I cannot say X percent etc.
The viewpoint of ghosts tends to be "romantic" probably due to the many Hollywood movies and fiction stories, where the ghosts have independent thought and an active post-death existance. That is not accurate. A ghost may be "reactive", but not "proactive". Plenty of spirits, including hostile ones that can perform limited physical world interaction, are proactive and capable of thought.
So the removal techniques that work for a ghost, which is really a form of echo or (alternatively) perhaps a "shell"; are not effective on a spirit which has the capacity of intent and at a low level, thought.