While a contact is on the banned list, they are not able to resubscribe to the lists you have banned them from. You can remove them from the banned list by following the below steps.
NOTE: Only the user that originally banned the contact has access to remove the ban.
Navigate from the home menu to Contacts.
Go to Ban a Contact.
Enter the Email Address of the contact you would like to remove from the banned list.
Select Remove contact from the "Banned" list.
Click Ban Contact.
The contact has now been removed from the banned list, however, they will not be automatically resubscribed to their previous subscribed lists. You can either do so manually, or allow the contact to subscribe on their own. To subscribe the contact manually, read Subscribe or Unsubscribe Contacts Individually.