I have the same problem. We have to face the fact that sometimes the chickens are smarter than us. Mine have a 30 by 60 area of their own, clipped wings, and still getting fence flyers. You would think with plenty of grass, constant food and fresh water they would stay put, but noooo, they are scratching up my flower beds, alongside the cats people keep dumping off. A fence would maybe work, but isnt very eye pleasing. Maybe those pinwheel things? The movement seems to startle other birds somewhat. Other ideas...
motion sensor water sprinklers
Some people claim sprinkling vinegar around the outer edges works, but it will kill the grass there. (i use it for weed on my walkway myself)
Others claim a layer of chicken wire laid down, then planting the plants works. They cant dig and for some reason dont like the feel of the wire under their feet.
Maybe one of these suggestions will help. Good luck and let us know what works (if anything)