Two ways...
- There are third party apps on App store that can do this for you. OR
- Add the phone number to you Contacts and block it.
Manage your blocked phone numbers and contacts
To see the phone numbers and contacts that you've blocked from Phone, FaceTime, or Messages:
Phone
Go to Settings > Phone > Call Blocking & Identification [Scroll Down to the bottom]
FaceTime
Go to Settings > FaceTime > Blocked.
Messages
Go to Settings > Messages > Blocked.
From these screens, you can add or unblock contacts or unblock phone numbers.
To add a contact from Phone, go to Settings > Phone > Call Blocking & Identification > Block Contact. Then tap the contact that you want to block. To add a contact from Messages or FaceTime, go to Settings > Messages or Settings > FaceTime, scroll down and tap Blocked, tap Add New, then select the contact that you want to block. To unblock a contact or phone number, swipe left over the number, then tap Unblock.