For a land line a call blocker is a programmable electronic box that plugs into the phone line between the wall and the phone. It also needs an electric outlet. For a smart phone a call blocker is an app. It may come pre-installed or you can download one.
For a simple cell phone ask your carrier. Or put the pest in your contacts list and give them the ring tone "no ring" or "silence". That will make the calls easier to ignore.