Does anyone have a cat with pica?

My 3 year old Siamese cat, Sofie, has a serious case of pica (pica is when one eats non food objects like dirt, metal, etc or low nutrition foods such a flour or baking soda). Sofie tries to eat plastic constantly. I have tried everything I can think of. I squirt her with a water bottle, scold her, spanked her (I hated to do that, but I am so scared she will choke!). I don't know what to do. Can anyone help?