How can I manually unsubscribe a user?

You can do this by searching for their email address under: Contacts --> Active Contacts --> Advanced search. 

Click on the cogwheel icon next to their email address and select "unsubscribe" from the drop-down menu. "Unsubscribe (global)" will unsubscribe them from all address books within your account, whilst "unsubscribe (address book)" will only unsubscribe them from the address book selected.