To manually unsubscribe a contact go to the Contacts tab on the left hand sidebar.

In the left drop down, select 'Email Address', leave the middle drop down to the automatically selected 'equal' and type in the full email address you wish to unsubscribe. Then click 'Filter contacts'. 

Next, click the contact icon on the left of the contacts name and this will take you to main page for the specific email address.

Here you will see the Contact Status, select unsubscribe on either Email, Direct mail or SMS to ensure the contact no longer receives your specified type of communication.

If a contact does not wish to receive any communications by Email, Direct Mail and SMS, then you must select all Unsubscribed tabs. 

You have now removed the contact from further communications.