There are multiple methods to unsubscribe a contact, if you would like to unsubscribe a contact from multiple audiences, easiest method is to use the search feature.
Click on the search icon (magnifying glass icon, top right of page)
Type a search-word(s) in the search box, and click on the Search button to find matching contacts across all your Audiences.
Select the contacts you would like to unsubscribe by clicking on the check box to the left of the contacts you want to update. (as shown below)
Click on the Action menu and select the option Unsubscribe.