I strung some bright green 50lb test fishing line every 3-4 ft around the edges of my pond. I strung it up on some of those steel fence posts. Heron have a large wingspan and will get discouraged the first time they get tangled up in it.
I got the idea from a nearby garden shop that had a large pond. They said it worked great and didn't detract too much from the scenery.
My Heron's did not return after that. I think I did hear some commotion the next morning coming from the pond, but nothing after that, and they didn't come back the next year either.