They will automatically stop receiving any campaign emails you send to them as soon as they unsubscribe. Our mail server checks outgoing campaigns against the opt-out list for you, and when you next update your email campaign history, you can see that the particular contact will not have received the campaign email.
Yes, they can click the same opt-out link on the campaign email they received, to be asked if they would like to opt back in. Alternatively, if they no longer have any of your campaign emails to do this, you can contact our support team where you can request for a recipient to be removed from the opt-out list. They will need to be contacted to confirm the re-subscription.