jam vs block off

jam is a noun but block off is not a noun.

Word	Noun	Adjective	Verb	Adverb
jam	Yes	No	Yes	No
block off	No	No	Yes	No

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

jam (noun)	block off (noun)
deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems
preserve of crushed fruit
a dense crowd of people
informal terms for a difficult situation

jam (verb)	block off (verb)
interfere with or prevent the reception of signals	obstruct access to
crowd or pack to capacity	render unsuitable for passage
get stuck and immobilized	block off the passage through
block passage through
crush or bruise
press tightly together or cram
push down forcibly