1) that doesn't help for spoofed numbers and 2) it's actually harassment if they don't have anyone on the other end of the line or keep calling despite being told that they must stop.
Modern cordless phone systems can block *any* number whether or not it's spoofed, fake, incomplete, or nonexistent. I don't know about cell phones. I've even got my own number blocked for those swindlers whose use mine as their caller ID. As for harassment, many phones and devices block numbers without even ringing. You don't have to ask them to stop calling - just block the number. It's not as hard to deal with as it used to be.