That would be difficult. Many people, including myself, always keep the cat when scrapping out a car, and I've scrapped many that I legitimately owned. If I can't sell it, I can always extract the PM's from it, and so can others. If you want to keep your cat, make it really hard to remove with simple cutting tools. Some people think it helps to paint it an odd color with high heat paint. I'm not sure about that last one.