Open the Settings app
Tap Messages
Tap Blocked Contacts
Tap Add New
Select the contact to block
To block a specific number from an incoming text: open the Messages conversation
Tap the contact name or number at the top
Tap Info (or the “i” button)
Tap Block this Caller
Confirm by tapping Block Contact
To block unknown senders: open Settings
Tap Messages
Turn on Filter Unknown Senders
To stop receiving from a specific thread without blocking: open the Messages conversation
Tap the contact name or number at the top
Tap Hide Alerts