In principle, what you want might be reasonable; in practice, I don't think it's entirely realistic to expect anything posted in a public location (whether on the internet or IRL) to necessarily disappear entirely after a set time (however long that set time might be). You can point out all sorts of reasons why it *ought* to be that way, but people have put forward all sorts of reasons why usenet cancels and supercedes *ought* to work, too, and they mostly don't, largely because in practice nothing public is enforceably deleteable. You can ask people to please not log it, but copies are inevitably going to exist anyway, IMO. Once you've said something in a public place, the feathers are out of the pillow and blowing around in the wind. This may be unfortunate, but I don't know any cure for it (other than to never say anything in public... for some reason that solution doesn't seem popular).

As far as things being taken out of context, I certainly can sympathize with that, but I would think the best defense against that would be to have the entire context at hand -- i.e., to log everything. (In practice, however, this also may be impractical; life is full of security tradeoffs.)