You might call your phone company and inquire about a service called "call blocking"

For a monthly fee - they provide a service on your line. When someone calls and you do not want them to call you again - you dial a quick number such as *69 and they will not be able to call you again.

However, it may not work for toll free numbers.

It would seem that the phone companies could do something about this - but I was told "we are a telephone company" - in the business of providing phone services. If no one is breaking the law - and they are paying the bill - the phone company will do absolutely nothing about it.