Double-glazing salesperson: "I just want to ask you - if you could have any room in your house double-glazed for free, which would it be?
Self: "None of them, but thanks for calling." [Click]
Financial services salesperson: "Do you have a couple of minutes to take part in a customer survey?
Self: "No, but thanks for calling." [Click]
Door to door religious salesperson: "Can I interest you in the word of the Lord?
Self: "No, but thanks for stopping by." [Click]
I find that the little gesture of politeness catches them momentarily off their guard and enables you to put the phone down or close the door with a clear conscience and without leaving an opportunity for comeback.