If you are trying to delete a sender email id and are unable to do so, first please check if this sender email id is being used in any existing active campaigns or not. If it's being used in an active campaign, then you cannot delete this, you will first have to delete all active campaign or wait for them to get over.
Each calendar invite is associated with a "Sending email id" and for a calendar entry to exist in recipient's calendar, it's important for the invite to be associated with an active sending email id. Hence it's not possible for you to delete a sending email id once it's used in an active and ongoing campaign.