Here's something that I've found effective in the past:
:you see the number calling in, answer:
"Darkshadow Consulting. Can I have your client ID #, please?"
"Um, what?"
"This is a private number for an IT consulting business that is only given to customers. We charge $125 per call. Do you have an existing client ID #? If you've been referred by one of our other customers, I'll be glad to set you up. What business did you say you were calling from?"
"Can't Stop Calling Insurance"
"Okay. And I have your phone number here. We'll get the rest of your info in a moment, I'm going to read a brief statement to you. Continuing this call, or calling this consulting number again constitutes agreement that you are authorizing that you will be billed $125 for each call for consulting services. Okay, we can proceed. Can I get your address, your name, and a direct number to reach you at?"
And so on. And then, if they keep it up, REALLY BILL THEM. I've never personally had to take it that far, but I've heard of large companies actually paying other people who also do this. In which case, the calls wouldn't be so bad, would they?