There are no sure things
I have seen pepper spray fail to faze a guy twice.
Time one was in police academy. One very large good ole boy in my class seemed completely unaffected by the stuff. He claimed his face was burning and he admitted to some pain, but he was still walking, talking and laughing afterwards.
The other time was when a mental case hopped in a squad car (the driver's seat!) and latched onto the steering wheel, and was shouting (apparently) passages from the Koran. He got hosed down most heavily. Didn't affect his behaviour one bit. By the time I got there he was handcuffed and shackled, reeking of capsicum and screaming "ALLAH AHKBAR!" at the top of his lungs, thrashing around.
Usually though it works MOST effectively.
The axe bites into the door, ripping a hole in one panel. The maniac puts his face into the hole, cackling gleefully, "Here's Johnny...erk."
"And here's Smith and Wesson," murmurs Coronach, Mozambiquing six rounds of .357 into the critter at a range of three feet. -Lawdog
"True pacifism is the finest form of manliness. But if a man comes up to you and cuts your hand off, you don't just offer him the other one. Not if you want to go on playing the piano, you don't." -Sam Peckinpah
"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." -Robert Heinlein