I don’t. There is this, and this, and this. Ultimately, the current behavior is documented, our users use diaspora* exactly that way and the large majority of users are fine with it.
Funny enough, none of them is federated. Do you want to un-send an email? Or un-send a text message? Un-send an XMPP message?
As @goob pointed out, you cannot compare federated systems with centralized systems for multiple reasons, and you shouldn’t even try. There is no permission system where you could toggle visibilities for some of your contacts as you wish at any time, and there never will be, as it is literally impossible to build that in a federated system.
I’ve explained that before, but I’ll happily do it again: diaspora* gives you the tools to decide with whom you want to share your contents. Once you click the share button, that’s it. You sent the email, you stamped the postcards and you threw your envelopes into the mailbox. Now, you could ask your recipients to kindly forget that you sent them that vacation postcard earlier, but let’s be honest, that’s not going to work out well.
Building a feature that somehow removes posts from other persons streams just because you decide you no longer want to share with them is asking them to forget you sent them the postcard, nothing more. There is no way to guarantee that the post is gone everywhere, and we most certainly don’t want to act this way. diaspora* is about giving users control about their stuff where we can, not acting like they have more control than they actually have. I hate to even think about suggesting more control than there really is.
Once you shared a piece of information with someone, you have shared that information with someone, and unless you’re part of the Men in Black, there is no unsharing of information.
And as for the other part of your suggestion; giving new contacts access to older posts: also no. Most of our users would not expect diaspora* to behave that way, and it’s clear by looking at current usage patterns. Ignoring that, even if we would decide to implement something like this, once again, we would have to re-deliver all past posts, which by itself is a really horrible idea. The amount of posts we’d have to federate is potentially infinite. The only possible way to sanely implement something like that would be to limit the amount of re-federated items, but that would make everything unpredictable for users.
You are, obviously, free to open a new thread under the Features and Ideas or Federation section, as I’m not going to tell you otherwise. Rest assured, I will comment the exact same response, and I know that I am not the only one with this opinion.