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.