It's also about posts appearing on search engines. All the search engines have bots in residence here that just report URLs back to the search engines, so they can analyse them and include them in search results. We can ban bots universally, but that wouldn't do anything unless we also restricted read permissions for guests: the bots could just come as unregistered guests and read The Fray anyway. The only way to stop posts from appearing in search engines that I'm fairly certain would work is by removing the read permissions of both bots and guests - we're only talking about The Fray, I presume, because it's sort of the 'private' forum, where we talk about ourselves and each other more than anything else. It's possible, but I have no idea if it would work, that we could individually ban the search engine bots (or just restrict their permissions so people can still find the forum on Google), but I suspect they'd just send different bots. Or rather, different bots would find us as they crawled their way through the web.
I understand the concern about 'private' posts appearing on search engines, I'm just not sure that there's anything we can do at all that would protect their privacy fully. If someone finds out something about you you'd rather they didn't know when they see something you posted here on Google, they could have found it out incidentally by registering had it not been on Google, anyway. Plus, this is all fairly anonymous if you want it to be - a lot of us know each other, have met each other, are Facebook friends, etc., but a lot of us are anonymous strangers to each other, too. Really, the only people who can actually find out who you are are we the moderators, because we can whois your IP address, but the information you can get from that is still normally pretty limited and we're supposed to be responsible enough not to abuse that power.