Open the Phone app
Tap Recents
Tap the info icon next to the contact or number
Tap Block this Caller
Confirm by tapping Block Contact
Open the Messages app
Open the conversation with the contact
Tap the contact name or number at the top
Tap Block this Caller
Confirm by tapping Block Contact
Open Settings
Tap Phone or Messages
Tap Blocked Contacts
Tap Add New…
Select the contact or enter the number
Tap Block Contact