Blocking contacts in iMessage - on iMac

I have blocked a sender successfully on my iPhone. However, on my iMac the messages still sneak through. I've opened Messages>Preferences>Blocked - and hit +. My contact list shows up. I click on a name - and nothing happens. The box gets highlighted in blue - but no names show up in it. There is no way to type into that box. My iMac and iPhone are generally synced - except for this.