I've been accused of worrying far too much about this issue, and perhaps I could dig up the threads where that worrying occurred. But I don't mind doing it again
It would be technically possible to shield the main page. The trouble is that doing so would prevent Eratosphere's main presence from turning up in people's search results. That would mean that people who have heard a little about it and want to look for it wouldn't find it in the normal way. It would also mean that fewer people would stumble on it. That might reduce the number of people who discover Able Muse and Able Muse Press, and I'm pretty sure that's a result Alex doesn't want.
Knowing how few people who join actually stick around to contribute to the conversation, I'd hate to see anything reduce the number of people who find us and look in and perhaps join.