Is turn off a verb

Yes turn off is a phrasal verb.

Turn off has a literal meaning and an idiomatic meaning.

The literal meaning is to stop the flow of something or to extinguish something eg

Don't forget to turn off the tape.

Turn off the light and put out the cat before you go to bed.

The idiomatic meaning is to stop listening eg

The boys turn off as soon as I mention homework.

Phrasal verbs can have the same forms as other verbs eg

turned off -- past

turning off -- continuous tenses