To delete your campaign, you will need to make sure your campaign is moved to the Free tier which will cancel your subscription. Do this by editing the Campaign Subscription Settings.
Next, on the Edit Campaign page, at the very bottom, click the big red "Permanently Delete" button. This will completely and permanently remove all data associated with your campaign and cannot be undone. Only the Game Owner has access to this button.
This will open one final modal with a confirm button. Please note, if your campaign is on a paid tier, you will instead see a link to Campaign Subscription Settings to downgrade the campaign. If you are on peasant (free) tier, you will have access to the red Confirm Delete Campaign button to finalize the deletion.