Rule reminder
Final conjunctions (affinché, perché meaning 'so that') and conditional conjunctions (a meno che non, purché, a condizione che, nel caso in cui) always require the subjunctive. 'A meno che' is always followed by 'non' and the subjunctive, even though the meaning is already negative.
We will proceed unless the client objects. Choose the correct form: 'Procederemo a meno che il cliente non _____.'