In case you have an email recipient for a campaign that would like to be marked as opt-out for this campaign, but decided not to click the link in the email you provided them, you can still mark them as opt-out manually.
1) Go to the Campaign in question
2) Find the recipient's Campaign Member record
3) Edit the recipient's Campaign Member record to denote that they'd like to mark that they've unsubscribed in this email campaign by checking the Unsubscribed checkbox.
4) Click the Save button
Since organizations have a variety of ways they track subscriptions, this process will simply mark that particular user's intention to be opted-out of this campaign. Unless you have created other triggers or workflows, this action won't actually opt them out from a newsletter, since you can use Soapbox Mailer for sending non-newsletters as well.
For understanding our best practices in subscription management, take a look at the following support article for further ideas on setting up your organization in Salesforce to handle this functionality: Creating and managing email lists.