In many cases, contacts unsubscribe themselves. But if you need to unsubscribe a contact manually, you can do this from the Contacts master list.
It's possible to unsubscribe a contact on an individual channel level, or a contact level. For example, if you have a contact who has both a subscribed email address and mobile number, you can choose to unsubscribe a single channel identifier—mobile number or email address—or you can unsubscribe the contact entirely, so they no longer receive any marketing communications from you on either channel.
To unsubscribe a contact:
Go to Audience > Contacts.
Select the checkbox for the contact that you want to unsubscribe.
Expand the MORE ACTIONS drop-down menu and select Unsubscribe from account.
Select the checkbox for the channel or channels you want to unsubscribe.
Read the confirmation message, then select OKAY.