Cut off … - See Exodus 31:14 note. The more full expression here used probably refers to some special form of public excommunication, accompanied, it may be, by expulsion from the camp.
Matthew Henry
Concise Bible Commentary
These verses repeat what had been said before, but it was needful there should be line upon line. What praises we owe to God that he has taught the evil of sin, and the sure way of deliverance from it! May we have grace to adorn the doctrine of God our Saviour in all things; may we have no fellowship with unfruitful works of darkness, but reprove them.