True, you can't catch something you never saw coming... But think about the average person. If you saw something you couldn't explain, you would probably hesitate or freak out. This moment of hesitation is crucial, whether it be to cast a spell or simply move out of the way... And perhaps stop was the wrong word choice. Slow the bullet by messing with velocity, or redirect it slightly... Chronomancy, while forbidden, could be used in this sense to slow the bullet itself or "return" the bullet to the chamber. Magic isn't necessarily all-powerful but when used creatively can accomplish things that might have before been thought to be impossible or impractical.
Those who are more aware of their surroundings would most likely sense something of that nature before it arrives. However, if we're talking snipers or something of the sort.... Well, you're out of luck, bye bye. XD Another good reason to stay hidden.
((Sorry about this long winded stuff. I'm in a particularly talkative mood. Feel free to tell me to shut up any time. XP))