Blocking a contact means that you will no longer be able to receive messages from that contact. This action can be reversed at any time by unblocking that same contact.
To block a contact, go to your Inbox and open your conversation with that contact. Then click on the Contact info icon on the top right corner of the conversation to open the Contact details window.
From the Contact details window, click the three dot Phone options menu and select Block Number.
To unblock a contact, follow the same path described above to the Phone options menu and select Unblock Number.