The easiest way to let contacts unsubscribe themselves from individual campaigns is by adding the %CAMPAIGN_OPT_OUT% tag inside an email in the campaign. This tag inserts a link that allows contacts to remove themselves from the campaign. The default text for this link when rendered in a live email is is "opt-out". 

If a contact clicks this link they will be permanently removed from the campaign. These contacts cannot be added back into the campaign, even if they're added through another action/trigger. The contact will still be subscribed, however, and other campaigns will continue as normal.

Another (more advanced) approach is to use conditions within a campaign to trigger "remove from campaign" action depending on how the contact behaves.

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