Yes, you can dehorn adult goats. The open horn roots will need more attention because of flies but it can be done easily with a wire saw. But why bother to dehorn them? Save the money and time and put them in the freezer. Then look around for another (disbudded) LaMancha doe to keep your doe company. You can get one in milk, or a dry yearling. Summer is here - Fair time! Graduating 4-H kids will be selling off some or all of their milkers. Many herd owners will sort out their show string and keep only their best milkers. You might get a nice LaMancha with the promise of the seller breeding her back in August or September. I wouldn't bother getting another wether, try to find another milker. Milkers make plenty of wethers! LOL
To avoid this goring problem next year disbud your goats at 7 - 10 days old just when you can feel the horn buds. Bucks develop the buds earlier than doe kids. It only hurts for a few minutes, and the benefits outweigh everything IMO. We had 100 dairy goats and Boers, and I disbudded all of the doe kids. The dairy buck kids I sold at 2 months through the local auction. Our ethnic buyers wanted them with horns and with testicles. It saved me work and they brought more $$$. Win, win! The Boer bucks were disbudded, castrated around 2 months old, and grown out for sale as project animals (or the freezer).
Horns injure children, other animals, tear up fences, and on one occasion a goat got her horns caught in the feeder and pulled it over on herself almost breaking her neck. I soon sold her, nice as she was. They make nice handles, but with milk goats you are milking they are so friendly and tame, you don't need anything but their collars to hold on to. Actually, instead of having to catch the goats we practically had to beat them off when we went in their pens! LOL