You can only delete subscribers when no email has been sent:
To remove these subscribers go to:
Paste in the email addresses of all the subscribers that you wish to remove.
If you don’t have the email addresses available to copy and paste, then you can get them by exporting them first, by going to:
Open the file, select all the email addresses in the relevant column, and copy them.
Delete all subscribers when you have already sent them campaign:
It isn’t possible to delete a subscriber that you have already sent a campaign to. This is because it would also delete essential data, such as your clicks and open rate statistics, and whether the subscriber had been unsubscribed.
If you have subscribers whom you no longer wish to send campaigns to, you can add them to the blacklist.