Unfortunately, once hemp borers get inside the stem, there isn't really a reliable non-chemical cure.
The best approach is to inspect plants frequently, remove and destroy any infested branches as soon as you spot them, and keep the grow area clean of plant debris. At the end of the season, remove all leftover stalks because the larvae can overwinter inside them.
For prevention next season, many growers use insect netting during moth activity, encourage beneficial insects, and monitor for eggs before the larvae bore into the stems. Once they're inside the stem, they're very difficult to reach without causing more damage.
I hope you can save most of your plants. Good luck, my friend! 💚🌱