For a Smart Phone, the phone may come with a Call Blocking app. Or you can download one
For a simple cell phone your carrier may offer call blocking. Ask them. Or put the pest in your contacts list and give them the ring tone "no ring" or "silence". That will send the call to voicemail without ringing.
For a land line you can buy a call blocking device. It plugs into the phone outlet and the electric outlet on the wall. Your land line phone plugs into it.