Once a campaign has started running there is no way to remove emails from the sequence. If there are emails that you would like to have not sent. Then from the main campaign screen you can pause the campaign by hitting the “Pause” icon on the sequence you would like to stop.

As a remedy, you can clone or duplicate the campaign, make the necessary changes then add the the contacts to the new campaign. Make sure to remove any emails from the sequence that have already been sent. 

Note: Once a campaign is started the only edits that you can make are to the subject line and body of the email. Outside of editing that text, you can not make and other changes to the campaign and would need to follow the steps above to change the campaign.