To manually Unsubscribe someone, open up their contact card and click Unsubscribe:
On this pop up, you'll find two options. One for "Unsubscribe" and the other to check a box to "Block Unsubscribe":
FOR 'SMALLTEAM' & 'ENTERPRISE' ADMINS:
As an added functionality, team Admins can enable the ability to sync "Unsubscribes" with "Email Opt Out" in Salesforce. You can find this under the "Admin Settings" page - NOTE: this feature will use Salesforce API calls
This is how the contact will appear in Salesforce: