Asteroids are very far apart from each other on the whole. Kicking one off in a different direction with a rocket isn't so different from a change in direction from a (rare) collision, except that it will almost certainly only affect one asteroid. ~25% of all the mass in the asteroid belt is the dwarf planet Ceres, which is only 0.0128 the mass of our moon. https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)