I was plagued by such people to the point that I considered having the landline taken out. Even the most rainbow coloured metaphores was not putting them off, unlike SWMBO who would give me the handset to do the honours, I handed out out some REALLY good and descriptive ones
. Until trebor informed me of BT Call Guardian a while ago
Now I am in telephone heaven, not a single nuisance call since, nil, nada, zilch, it has blocked 100% of them. Anyone who calls for the first time has to leave their name on the message system for the phone to ring, I then accept once, always, or never. The phone displays how many calls it has blocked since the phone was last used eg (2X), it resets when you use the phone. It was high at first (9X), now it just displays the odd one. I cannot recommend this highly enough.
I had 2 or 3 on my mobile phone during the past month or so, following adding a phone number to online services I registered with being required, that I decided to do (even though I tick the little do not share box), I blocked the numbers and they stopped.