Use ～てはならない for absolute prohibitions in written or formal registers — contracts, company rules, compliance notices, policy documents, and speeches. It is markedly stiffer than ～てはいけない and carries the tone of a rule imposed from above rather than a personal warning, so it rarely appears in casual conversation. In email and meetings it surfaces mostly when quoting regulations or stating principles (「～てはなりません」 in polite speech).