How does Monty handle opt-outs and 'not interested'?

Immediately and permanently: when a lead asks to stop hearing from you — explicitly or in substance — Monty marks them do-not-contact, halts all scheduled follow-ups on the spot, and sends nothing further to that address. The do-not-contact list is visible in Settings → Do not contact and you can add anyone to it manually; opt-out status is also re-checked at the moment of sending, so nothing leaves to an opted-out address — in either mode.