I know it doesn't make much sense, but most things on LOST don't at first. Nemisis killed Jacob by kicking him into a FIRE. Jacob died there and would have most likely turned into ash. Perhaps Jacobs ash is a repellent to the Smoke Monster, but this would involve his ash traveling in time to be used before.
I don't know I feel like its too much of a coincidence that Jacob died in a fire and the Smoke Monster hates ash.