Login to your service provider. Find the list of calls. Click or right click on the call and select block from menu list. After that, their attempt to call does not even register trying to call. They get very upset. Hurt them where it huts the most by blocking them.
Usually I don't answer these but was bored and sometimes enjoy playing their own games at their expense. So this time it was Direct TV (not!) offering me a special deal. I have DTV so I said oh yeah, someone already called me and I'm going to pass. He muttered thank you for answering phone and hung up. He'll toughen up or get fired. Too nice for the shark tank.