head off vs prevent

head off and prevent both are verbs.

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

As verbs, prevent is a hypernym of head off; that is, prevent 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
prevent: keep from happening or arising; make impossible

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

head off (verb)	prevent (verb)
prevent the occurrence of; prevent from happening; to protect from or to keep away anything undesirable; to ward off	stop (someone or something) from doing something or being in a certain state
	keep from happening or arising; make impossible