Most Samsung smartphones come with both call and sms blocker. And they're not expensive phones. My current one cost ?30 from the shop, and the previous one cost ?20 from a car boot.
Now to the topic: I simply ignore them. Don't get into any justifications, explanations or arguments with them, as this would only wind them up and make them even more obsessive. Plus any arguing or explaining with such individuals tends to be never-ending.