In rare instances, contacts may abuse your service by sending spam, inappropriate or offensive messages. You can block these contacts by navigating to the "contacts" tab, selecting the checkbox next to the contact’s name, and clicking the "Block" icon. All incoming messages from blocked contacts will be archived.

You can 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 "Activate" button. 

Up Next: Search for Contacts