You have a rock river?
Send it in .... You'll ruin the nut trying to turn it. Or get some cotton rags, wrap the rags round the castle nut and drizzle nail pollish remover over the rags till it dripps. Flip and repeat. wait twenty min and make shure the rag wrap stays soaked the ENTIRE time. Then unwrap and try to unscrew it. My results were 50% I had to ship one back, and I got one loose. Good luck.
use thin strips of rags, and be careful. paint finnishes can be damaged.