# That person must be cut off

"That person must be sent away" or "you must send that person away". Here the phrase "be cut off" means to be disowned and sent away. See how you translated this in Numbers 9:13.

# the water for impurity was not sprinkled on him

"no one sprinkled the water for impurity on him" 

# the water for impurity

"the water that is sprinkled on impure things to make them pure" 

# He will remain unclean; his uncleanness will remain on him

These two phrases mean basically the same thing and are combined for emphasis.