The word agent really shouldn't be used any more, because it's a word that gives them some sort of credibility.
It should just be replaced by the word parasite.
If you look up the word parasite, it fits the job description of people like this almost perfectly.
"a person or thing that takes something from someone or something else and does not do anything to earn it or deserve it."
"something that resembles a biological parasite in dependence on something else for existence or support without making a useful or adequate return."