Perhaps the majority of organic compounds present in an aquarium will contain nitrogen. Proteins, especially, are loaded with nitrogen, with every amino acid having at least one nitrogen atom and some have 2, 3, or 4 (arginine has 4).
GAC is a good way to remove many organics, and should reduce DON. Skimming may also work very well.
I'm not a fan of ozone, but it should work to reduce DON. The nitrogen-containing organics may be more likely to be consumed by bacteria after being broken up a bit with ozone.