How do I scold my chihuahua for pooping?

I understand that if you see a dog about to poop, you can scold him then and there. But what if he did it a while ago maybe. How do I scold him? I can't say noooo and whip him because then he will not know what i am scolding him for. Please Help!! He poops in my mom's room but I cannot stop it because he just doesnt understand.. any suggestions????