We used to get a lot of "survey" people calling...when we would tell them we don't want to be involved they say, 'oh please this will only take a few minutes!"
I don't know what world they are from but their 'few minutes' are different than mine.
What we started doing is to say "just a moment please' and they say 'sure'...then we would put the receiver down and walk away and go watch tv or what ever.
Eventually they get tired of waiting and hang up. If they call again I do the same thing and finally they stopped calling completely.
Some just do not understand the meaning of "no" but waiting on a lifeless phone call seems to strick a cord.