In rare instances, contacts may abuse your service or send offensive messages. You can block these contacts by navigating  to the "contacts" tab, highlighting the contact, and clicking the "Block" icon. All incoming messages from blocked contacts will be archived.

blocking contacts

You may unblock a contact at any time by clicking on the "Blocked" sub-category underneath the "Contacts" header, clicking the box to the left of their name, and clicking the "Unblock" button. 

blocked contacts