1. To block a number, from the home screen, tap Messages.
   
2. Tap the Desired message thread.
   
3. Tap Details.
   
4. Tap the Info icon 'i' next to the contact or phone number you want to block.
   
5. Scroll to the bottom of the Info screen and then tap Block this Caller.
   
6. Tap Block Contact.
   
7. To unblock, tap Unblock this Caller.
   
8. You can see the contacts and phone numbers that you've blocked in Settings under Messages > Blocked, Phone > Blocked, or FaceTime > Blocked. You can also Add New blocked numbers and Edit (Unblock) numbers from within Settings.
   Note: Learn more from Apple support article: Block calls and block or filter messages on your iPhone.