I stopped being less-than-firm about it a long time ago. If I get calls for charity or for sales, I politely interrupt and say "I'm not interested" or "I've already done my charity donations for this year" (which, really, are more volunteer work than money I don't have), and then I hang up without letting them continue. They are trained to take advantage of politeness and unwillingness to hang up or interrupt. So I do those things politely and firmly.