Can't do warning shots with bear spray.
I have read a few accounts of bear spray being used to stop an attack. One fellow told how the bear had him and he could not get to his spray but he was fortunate that a companion had spray and was able to stop the attack.
I spoke to a woman that had survived a vicious attack while hunting with her husband. She wanted to buy a bigger gun than the .270 she was carrying. I asked her if she tried the 270 and she told me that she was not aware of the bear till she was on the ground and the bear was doing her thing.
Whatever one chooses to defend against an attack, I think that one should be familiar with your equipment. A spray can or firearm I think a bit of dry point and shoot would be a good thing.