Hi, the title says it all really.
I've just come back from a day off to find that somebody removed a load of stories from the sprint and replaced it with a different set. Nothing really wrong with that, except that the new stories had no estimate against them. Consequently the burndown looks a complete mess.
Is there any way to prevent stories being added to sprints unless they have a Story Point estimate? This seems fundamental, as no story should ever enter a sprint unless it has been estimated; and the burndown simply doesn't work properly with a null estimate (as opposed to "zero" estimate).