this is not a solution; it is a completely wild-and-crazy idea, but…
… an interesting approach to this problem might be a configurable ‘last-data’ time for a goal. so if i tell beeminder ‘stop accepting data after 5pm,’ beeminder doesn’t let me add new data for the day, nor does it update any existing points.
i can already think of the support nightmare that would probably be, but in fantasyland it would be a nifty way to get around these sorts of limitations for those who really really want the hardest possible stop.