Usually, your subscribers can subscribe or unsubscribe by themselves. However, it is possible to unsubscribe one or more contacts manually.
To unsubscribe subscribers from one or more contact lists:
Browse to the main "Contacts" menu.
Click on the contact list in wich you want to remove one or more subscriber.
Browse through the secondary menu "Remove Subscribers" on the right.
Enter each email address you want to remove on a separate line.
Click on the "Unsubscribe" button.
Unsubscribing from several contact lists at the same time:
If you have more than one contact list and you want to remove a subscriber thru all your lists, make sure you select the option "Unsubscribe these email addresses from all my contact lists".