After pressing the Send button in Soapbox Mailer campaign, you might find yourself needing to quickly cancel the send for some reason. While Soapbox Mailer can't retroactively delete an email it's already sent, you can stop it from sending future emails that it has queued up in the campaign. Here's how.
1) Login to Salesforce and go to Setup > Administration Setup > Monitoring > Apex Jobs
2) On the Apex Jobs page, look for any jobs that have the Apex Class value listed as "UpdateCampaignMembersEmailBlast", which will be the Soapbox Mailer campaigns you've sent
3) Look for the job whose status is "Processing" and whose "Apex Class" value is "UpdateCampaignMembersEmailBlast", and click the "Abort" link in that row
That's it! That will cancel the email campaign in question. You can go back into the Campaign record now, update what needs to be updated, reset all your Campaign Members to have a status of "Targeted" (if you want to send to all the people you've already sent to and to people you haven't already sent to), and then send again.