Open the Messages app
Open the conversation from the sender you want to block
Tap the contact name or number at the top
Tap Info
Tap Block this Caller
Tap Block Contact to confirm
Open Settings
Tap Apps
Tap Messages
Tap Blocked Contacts
Tap Add New
Select the contact to block
Open Settings
Tap Phone
Tap Blocked Contacts
Tap Add New
Select the contact to block
Open Settings
Tap FaceTime
Tap Blocked Contacts
Tap Add New
Select the contact to block
Open Settings
Tap Messages
Turn on Filter Unknown Senders if needed