There are two ways to unsubscribe people from your newsletter. You can either unsubscribe subscribers individually or unsubscribe in bulk.
In order to unsubscribe an individual, you can go to Subscribers and then toggle off next to their name. In order to unsubscribe in bulk, click 'export' to download .CSV file. The file should download with seven columns: first name, last name, email, active, subscribed, created and updated. Those that you would like to unsubscribe should have a 0 in the active column. Save the .CSV file and then re-import into the platform.
This video will walk you through unsubscribing in bulk.