| An expression meaning "with no exceptions"; the Oxford English Dictionary first cites it in 1866, although use of the word "bar" as a preposition to exclude something go back two centuries earlier, deriving from the use of physical bars to prevent entrance to or exit from a building.
The Bar None was also a candy bar made by Hershey's Chocolate, first produced in 1986. The original version was layering of chocolate wafers, chocolate creme and peanuts, and came in a black and brown wrapper. In 1993 the bar and its packaging were changed; caramel was added and the wrapper became yellow. The bar stopped being produced at all sometime after 1996 when the nutritional information below was published in a CNN comparison chart.
Nutrition info for the later version of the bar (serving size 1 43-gram piece; there were two pieces in the package):
Sources:
http://dictionary.oed.com/cgi/entry/00017428/00017428se1
http://www.swlink.net/~danichi/heydude/otherbn.html
http://www.cnn.com/FOOD/resources/food.for.thought/sweets/compare.candy.bar.html | 
