An interaction designed to suck money out of you is not governed by the same social rules as other interactions. It's a whole 'nother dynamic, and has its own conventions you can learn, practice, and employ brilliantly and responsibly.
Their motives are not, as you feel, designed to control you-- or organize you-- or do anything personal with you at all. They just want MONEY. So-- if they aren't going to GET any money from you, you are actually HELPING them when you make the contact as brief as you possibly can. Smile in good conscience and send them packing. You will not be behaving in a manner counter to your best intentions, if you just rmember you are helping them move on to someone who may give them what they seek.