Ibn Al Qayyim (rahimahullaah) wrote: 

“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.”