Forming a passive and negating it involve two independent processes: modifying prefix/stem vowels for voice, and adding لَا for negation.
يَضْرِبُ → يُضْرَبُ → لَا يُضْرَبُ — he strikes; he is struck; he is not struck.
يَسْمَعُ → يُسْمَعُ → لَا يُسْمَعُ — he hears; a sound is heard; a sound is not heard.
A natural full clause:
Word / formMeaning / explanation
يَكْتُبُ الرِّسَالَةَhe writes the letter.
تُكْتَبُ الرِّسَالَةُthe letter is written.
لَا تُكْتَبُ الرِّسَالَةُthe letter is not written.
Turning a whole clause passive can also alter the grammatical role of the associated noun; the morphological operation itself changes the verb's vowels.