When creating a new email campaign, one of your first steps is to determine who should receive the email. This is typically done by creating a report in Salesforce that has filters built into it to include only the correct people in the campaign. Since Soapbox Mailer is meant to be a flexible tool for sending emails of all kinds, it does not include a pre-built system for managing subscriptions. This allows your organization to manage subscriptions as you see fit, through a variety of data architecture choices.
Assuming you've followed our best practices for list management, one easy way to exclude people from campaigns is to filter by a picklist subscription field. Then, assuming you've updated the Contact records already, you can create the report to only include Contacts that have that particular picklist field set to "Subscribed".