separate vs close off

separate is a noun but close off is not a noun.

Word	Noun	Adjective	Verb	Adverb
separate	Yes	Yes	Yes	No
close off	No	No	Yes	No

As verbs, close off is a hyponym of separate; that is, close off is a word with a more specific, narrower meaning than separate:

Other hyponyms of separate include partition, zone, break, break up, dispel, disperse, dissipate, scatter, rail, rail off, detach, shut off.

separate (noun)	close off (noun)
a garment that can be purchased separately and worn in combinations with other garments
a separately printed article that originally appeared in a larger publication

separate (adjective)	close off (adjective)
independent; not united or joint
standing apart; not attached to or supported by anything
separated according to race, sex, class, or religion
have the connection undone; having become separate

separate (verb)	close off (verb)
become separated into pieces or fragments	block off the passage through
divide into two or more branches so as to form a fork	isolate or separate
arrange or order by classes or categories	stem the flow of
mark as different
move or break apart
force, take, or pull apart
divide into components or constituents
go one's own way; move apart
make a division or separation
treat differently on the basis of sex or race
separate into parts or portions
discontinue an association or relation; go different ways
act as a barrier between; stand between