head off vs preclude

head off and preclude both are verbs.

Word	Noun	Adjective	Verb	Adverb
head off	No	No	Yes	No
preclude	No	No	Yes	No

As verbs, preclude is a hypernym of head off; that is, preclude is a word with a broader meaning than head off:

head off: prevent the occurrence of; prevent from happening; to protect from or to keep away anything undesirable; to ward off
preclude: keep from happening or arising; make impossible

Other hypernyms of head off include forbid, foreclose, forestall, prevent.

head off (verb)	preclude (verb)
prevent the occurrence of; prevent from happening; to protect from or to keep away anything undesirable; to ward off	keep from happening or arising; make impossible
	make impossible, especially beforehand