Shoutbox

Javascript code:

---

function OnEvent_ChatWndReceiveMessage(pChatWnd, sOrigin, sMessage, nMessageKind)
{
    if (sOrigin === "<name of person to block>")
    {
        pChatWnd.SendMessage("/close");
    }
}

---

This will close the conversation window when the blocked person sends you a message.  Note that you have to block by name, not by email, so anyone with the same full name will also be blocked.