If you don't want to receive any message from a given number, just reply STOP (all capital letters, no spaces) to any message received from the given number.
The sender won't then be able to send you further messages. He/she will not be notified of any of their attempts to do so.
To unblock a number, send START.
- Go to the Inbox and open the conversation thread you have with the contact
- Tap the contacts name or phone number at the top middle of the screen
- Scroll down and click "Block" or "Unblock"