The whole point is that even if you prevent them from buying high risk securities, they will buy AAA rated commercial paper from ibanks that will. And surely y

The whole point is that even if you prevent them from buying high risk securities, they will buy AAA rated commercial paper from ibanks that will. And surely you're not going to prevent a bank from making loans, that's their whole raison d'etre (safekeeping of money is incidental). Short of getting rid of central banking, I don't think you can do anything about this.