In each contact's form (Contacts > Select a contact) you will find a "Campaigns" section, which also serves as an "unsubscribe" section. This section displays a list of all the campaigns to which a specific contact has access.
The contact can unsubscribe himself via his public profile or can be unsubscribed by the account administrator.
The administrator can unsubscribe a contact by clicking on the X next to the campaign. The status will be updated and the contact will change from subscribed to unsubscribed.
The contact can choose to unsubscribe from some campaigns only or from all campaigns. This option can be changed in the subscription management in the administration.
Unless the contact is unsubscribed from all campaigns, the contact will be automatically subscribed to the newly created campaigns.