Actually In backgammon it is frequent that a position is reached when one player cannot lose, even if (s)he rolls 2-1 and the opponent 6-6 for the rest of the game. Obviously this is only applicable if there are no overlaps and therefore no chance of capturing. In these cases you should resign unless both players are happy to keep chatting. It is really only tournaments that I would apply this to since a lot of other players may be waiting for the outcome before proceeding.