I think one of their clan members reported it. But I don't think that is going to help too much since the person was using a mule account to post it. Just have to hope that they don't continue doing this, there isn't much anyone else can do.
I looked into the situation in the other clan's thread, realizing that he was just doing it because he was declined membership to the clan. One of the members apologized for what was going on and I accepted their apology.