Complete dehorning, as in removing the horn button manually by cutting or by burning, needs to be done when the calf is fairly young. A couple of days old isn't too young. Six months may be.
If it is weaned, you may have to settle for cutting them. When they are still small, say 2" or less, I have trimmed them with a PVC pipe cutter. Just trim as close to the head as you can and be prepared for blood squirts. If older, I put them in a headgate and use a sawzall. I've had to cut back the horns of one of my cows three times now. They curve back towards her ears and when they get too long they rub on the ears.
Ken S. in WC TN