There is not a specific function on the backend of Sphere to unsubscribe supporters from specific email campaigns.

As a workaround you can request that the supporter click the link to Unsubscribe from this email that is listed at the bottom of most emails to unsubscribe themselves or, if the supporter sends the email to the organization they can click on the link and fill out the unsubscribe information for the supporter to unsubscribe them. NOTE: The email address used to Unsubscribe must be the email address the email was originally sent to by the Sphere system. It cannot be the forwarded email address if that applies.