Karma is action and consequence, cause and effect. We are not punished or rewarded for our actions. We merely experience their consequences.
One could say that if water is poured on the floor, the floor gets wet for that reason. But there is no deliberate intent on the part of the water, the floor, or even the person who spills the water for the floor to get wet. It is just the direct, natural consequence of the action.
However, causes require specific conditions for their manifestation, and if conditions are unsuitable, causes cannot manifest. For example, the most potent seed still requires earth, moisture, warmth, and air. If it is kept in a closed, dry, empty box, it will not sprout.
Therefore, karma can be prevented from manifesting (by removing the conditions for its expression), and it can also be purified (by removing the wrong view of self and other). It is not unavoidable. It is not destiny.