You need a call blocker.

For a land line phone a call blocker is a programmable electronic box that plugs into the phone line between the phone and the wall.  You can buy one at Best Buy or on Amazon.  Most smart phones come with a call-blocking app or you can download a free one.

For a simple cell phone ask your carrier or put the pest in your contacts list with the ring tone "no ring".  That won't stop the calls but it will make them easier to ignore.