When this word first turned up, I wondered if it was a typo for ‘prescriptive’. But no, it means forbidden. Later in the text, it turned up again several times as the verb form, proscribe.

• Oxford: forbid, especially by law; denounce or condemn; outlaw.
• Collins: to condemn or prohibit; to outlaw; banish; exile; (in ancientRome) to outlaw (a citizen) by posting his name in public.

That final meaning in the Collins definition, of posting a person’s name in public, is where the word comes from. In Latin, pro (before) and scribere (write). Etymonline explains it well.