Joe,
I know it won't help you this year, but a purple martin house could possibly do the trick.
I was thinking about other natural enemies of the herons, and all I can think of are alligators, snakes, and hawks. Too bad you can't get the above in mechanical versions. All they would have to do would be to move just a little, and the herons would panic. Or set up something to automatically open your back door every 30 minutes.
