If you take the approach of policing individual sender accounts with a strict anti-abuse policy, you have a chance of succeeding. I'm sure you have already discovered that the moment you allow anyone to sign up for an email sending account, the worst of the worst actors immediately take up the opportunity to do so! Cloudflare has a massive amount of data about web traffic and I would hope that this data can be recycled into effective threat detection and control. No doubt you already know this and have people working on it. Good luck!