Hi, what is your current version of Kentico?
Because in Kentico 8.2 this functionality is working out of the box. If you subscribe a contact group to a newsletter and some subscriber inside of that group decides to unsubscribe, the {%UnsubscribeLink%} macro will unsubscribe only that subscriber but will keep the rest of the contact group members subscribed.
Here is how it works: When a subscriber clicks on an unsubscribe link, an activity called "Email campaign unsubscription" is logged for that subscriber. Then, if another newsletter issue is sent, the system filters out every subscriber that has this activity logged among its activities.
And of course, if you deleted the "campaign" activity from subscriber's activities, he would start receiving newsletter issues again.