Repel the thought, for if you don't, it becomes an idea. So repel the idea, for if you don't it will become a desire. So fight against that(desire), for if you don't, it will become a determination and a passion. And if you don't repel that, it will become an action. And if you don't replace it with its opposite, it will become a constant habit. So at that point, it will be difficult for you to change it.