NOT A TEACHER
'Off' is an adverb or, more correctly, an adverbial particle.
Phrasal verb = verb + adverbial particle
You always have to put a pronoun (like 'it', 'him', etc.) between the verb and its adverbial particle. In other words, you say 'leave it off' but not 'leave off it'.
If it is a noun, you can put it inside or outside the phrasal verb, e.g. 'leave off the xml prolog' or 'leave the xml prolog off'.
Hope it helps.
Student or Learner