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.