After receiving a call from this number numerous times a day, I finally answered. No one speaks up. After saying Hello three times, I finally said "This is your last change to speak up. Otherwise I'm blocking your number". Still no response. Number is officially blocked.
Is it an iPhone? If so, find the number in your "recents" list. Hit the info button to the right of the button. Scroll down to the bottom. Select "block this caller".