"cast the unclean spirit out." This refers to casting the spirit out of the boy. AT: "cast the unclean spirit out of the boy" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
# This kind cannot be cast out except by prayer
The words "cannot" and "except" are both negative words. In some languages it is more natural to use a positive statement. AT: "This kind can be cast out only by prayer" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])