So, let's take case number two. You ask someone on a date. He or she does not know whether or not they're interested but figures it's worth giving a shot. He or she says yes. After the date, however, said askee has become certain: definitely not interested. By virtue of having said yes to one date do they now owe the asker a full breakup spiel? Do they owe any explanation whatsoever? No.

The blow-off is a fully acceptable maneuver at this point. It should always be done nicely, of course. With a smile. But saying no to dates after having gone on one date (and maybe even two or three, really) is no different than saying no from the beginning. No explanations required.