I remember once about a year ago a telemarketer called me asking if I was interested in buying books for our children.
I responded quickly, "I'm sorry, our children are blind."
They were completely stunned, said sorry, and hung up.
My wife was laughing hysterically (our children can see fine, and she loathes telemarketers).
I'm usually very kind to those people, as I don't think anyone chooses telemarketing as a career. I think it's something they do to make ends meet.
Usually when one calls me, I just say, "Sorry, I'm not interested, but good luck with your calls today!"
They are usually quite surprised, courteous, and end the call right there, saving us both time.
People are people, even telemarketers.