So to get Amazon to police their customers either requires regulation or an external economic pressure. Blocking AWS from folk's mail servers would apply some pressure,
No it would not. That is what AWS wants you to to.
making areas of the net go dark to AWS would apply more pressure faster. A considerable amount of pressure could be placed by a big enough money damages lawsuit but that has a feedback delay of months to years.
And such lawsuits can go both ways. As soon as a company moves beyond protective blocking of port 25, to punitive blocking of all traffic from AWS, they run the risk of being the target of a damages lawsuit. Not to mention complaints from their own customers. There simply is no simple solution to this problem. --Michael Dillon