Years ago, I found it necessary to start maintaining a list of Amazon’s subnets so that I could block them easily. This list can be used in .htaccess and firewalls that can block access using CIDR subnet addresses.
This topic was formerly hosted on the forumpostersunion.com website, which now appears to be gone. The AWS Forums and AWS re:Post threads are also obsolete.
For current information, see: Amazon EC2 Public IP Ranges