One idea is to put more pressure on writers of letters of recommendation. There is obviously already some rather weak "reputational incentive" to not give letters of recommendation to fraudsters. But it could be made a more formal policy. Letters could be required to include a statement about the candidate adhering to various formalized "good behavior" policies. Then when fraud happens there could be actual consequences for the letter writer. This would be like the culture in software development where you don't blame people for writing bad code, you blame reviewers for letting in bad code. (Idea comes from Angela Collier, in relation to other forms of antisocial behavior).