had a row with a double glazing firm some years back about this, when I moved out, I got my parents to go ex-directory to help stop scams etc, whilst visiting the phone rang, when I stopped the guy mid speech to ask how he got the number he said their computer dialed numbers at random till someone answered, so,
I got the name of the company, the name of the person I was speaking to, then informed him that a, we were not interested in any of their products, and b, could he NOT ring again, and that if he did ring again I would report him for making nuisance phone calls, he reckoned he couldn't guarantee that because it was the computer that dialled the numbers, when I said that was not my problem but his, he got all sheepish.
Look in the phone book or yellow pages, under terms and conditions of having use of a telephone line or system, a nuisance phone call is classed as anyone who calls after you have asked them not to, and a complaint can be made, strangely enough, they never rang back!!
Used this method a few times and it's always worked!!