Tom Smith's study on bear spray effectiveness documented 2 cases where bear spray was used against polar bears; both successfully. It didn't sound like the bears were being aggressive at the time the spray was deployed though, but rather it was used to deter curious bears. The study stated:

In both polar bear incidents, subadult bears approached humans in a pickup truck there to observe bears feeding on bowhead whale (Balaena mysticetus) remains near the village of Kaktovik, Barter Island, Alaska, USA. In both instances (100%), bear spray stopped the bear’s approach and turned the bear away. Neither of these bears returned to the truck following spraying.