The email opt-out in Sheep acts as a system wide block. Since GDPR and the requirement for contacts to opt-in we recommend only using the opt out setting for contacts that really don't want your communications. (At absence of an opt-in should be sufficient for most and is less drastic.)
On a person, click the edit button on the Contact card to bring up the edit form. Use the switch at the bottom of the form to change the value.
Note: Transactional and templated emails can still be sent when the opt-out has been set.
How it works with MailChimp
Switching on email opt-out will prevent contacts from being pushed to MailChimp and will unsubscribe them from any lists that they are subscribed to.