Steve if they are as clever as you say, it sounds like someone needs to invent a heron trap. You catch them alive and then take them somewhere they can't find their way back from.
Mitch
PS
I also have a problem with koi getting scratches on their head. I have these large, neat looking rocks that stick out over the water a good ways in some cases, and I also think the pond looks best when it is really full and some of the rock are below water level. You can imagine what happens in feeding frenzies. Aka Mats are very white under brilliant orange head color and the scars heal black.