I'd like to know the meaning of "to cut him off" and "there" in the following excerpt from the chapter Five of The Great Gatsby. Does "cut him off" mean "to interrupt him" or "to put a stop to his offer"; does "there" mean "there and then"?
“Well, this would interest you. It wouldn’t take up much of your time and you might pick up a nice bit of money. It happens to be a rather confidential sort of thing.”
I realize now that under different circumstances that conversation might have been one of the crises of my life. But, because the offer was obviously and tactlessly for a service to be rendered, I had no choice except to cut him off there.