。
She was forbidden to smoke.
Literal
She [topic-は] smoking [object-を] was-forbidden.
禁じる ('to forbid, to prohibit') is the somewhat formal verb for prohibition, more written and weighty than the colloquial だめにする or やめさせる. The passive 禁じられた frames the speaker as the recipient of the prohibition rather than naming who imposed it — a typical Japanese move when the agent is unimportant or generally understood (a doctor, an authority, the household). The kanji 禁 carries an air of officialdom: it's the same character on signs reading 禁煙 ('no smoking'), 禁止 ('prohibited'), 立入禁止 ('keep out'). Smoking has been increasingly restricted in Japan since the late 2000s, with many restaurants, stations, and public spaces now smoke-free.