Here "cut off" is a metaphor for separation from Christ. AT: "You have ended your relationship with Christ" or "You are no longer united with Christ" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Paul is speaking ironically here. He actually teaches that no one can be justified by trying to do the deeds required by the law. AT: "all you who think you can be justified by doing the deeds required by the law" or "you who want to be justified by the law" (See: [[rc://en/ta/man/translate/figs-irony]])