Legal Question in Criminal Law in California

Of course not. How do you propose you would 'regulate' or prevent him if he was at his second story window?. Would you call the police? How would they prove he was 'staring illegally' if it were actually a crime? Would you take pictures of him doing so to give to them? Wouldn't you then be seriously violating HIS privacy?

If you don't want him staring, tell him that, politely. If you're obsessed about it, you could raise the wall height.