Since Feedburner became a Google service, it’s no longer possible to delete your account. All you can do is remove all your feeds and disable all additional features. The Feedburner service remains active for your account unless you delete your entire Google account.
If anyone knows of a way to remove your Feedburner account completely from your Google Account, please let us know. For now, it remains greylisted, since it doesn’t really store any personal information.