I think the best would be if Arcana, on behalf of the guild asked the site to remove all maps that aren't credited. If they will not comply - or says something stupid like "proof it", we should send out a mail to all members and tell them to go look (and we can do that on DA too), and perhaps drop a word to wizards as well. But let them have a chance to make emmends first, it is after all a rpg community