I think realistically the only way you could enforce this is to legally required registration of job adverts with the government (you register the advert, you receive an ID; anyone advertising jobs without a valid ID is heavily fined), and then also require companies to register the outcome of the advert (internal hire, external hire, withdrawn, etc.).
Then it would be possible to actually identify suspicious behaviour, and you could publish stats about companies' hiring practices so candidates can avoid them etc.
Then it would be possible to actually identify suspicious behaviour, and you could publish stats about companies' hiring practices so candidates can avoid them etc.