If you'd watch the replay, you could see that the spectator had connected around where

KingLx25x had quit your match.
In regards of making sure mm can't be spec cheated. Playing in MM already mutes the lobby and spec messages, I suppose that the next step would be passwording games, or locking spectators out of them in another way. But that would require hosts to do something about it.
Generally moderators would inspect if any cheating could take place. In this regard, the accusation seemed to be that of intentionally creating a rover mirror... which to me doesn't really make sense as rovers are a good choice for the map anyway.
I wish you great future games, you played well.