How do you teach an 18 month old not to sass you, and to be nice/gentle?

I have an 18 month old son, who seems to like to sass me when he is told not to do something, and when he is told to be gentle/ nice to our cat, he literally throws things! I don't want to spank him and the thought is completely revolting to me! I myself was an abused child. I really don't want this to be the same for him! I am the only one home with him for 8 hours a day while my husband is out working. I have tried tons of things, stop short of tying my son down to keep him out of things.