Then you're enforcing. That's enforcement. If you want someone to register and they won't, they don't want to, they think they can get away with it and that's their mindset, they won't do it.
Then the question is around trying to catch that person, because you think someone's going to rat them out, and then you follow up with enforcement. However, no piece of legislation is going to force someone to tell you something they don't want to tell you.