We used to have moderation turned on, but it meant that there was a delay when new users posted to the group and they often ended up sending multiple messages.
Unfortunately some bogus messages still slip through google groups' spam filters. When that happens I report the message as spam, delete it, and ban the author - hopefully this will eventually feed back into the spam filters. If the situation gets worse we could look at turning moderation back on, but I'd prefer to avoid that if possible as imho it makes the group feel less open/welcoming.
In the meantime, please don't reply to any of those messages.