When a user unsubscribes directly from an email it's a one-click deal. Subscribers can also manage which of one or more lists they are on by logging in and then clicking the Subscribe link.
From a broadcast email, the one click unsubscribe will add the email address to a Black List specific to that website.
Any future attempts to add emails to an email broadcast from any source are filtered against this black list, to ensure you don’t accidentally breach privacy/regulatory requirements for sending email.
The only exception will be if a user explicitly re-subscribes - at which time they will be removed from the black list.