We don’t delete accounts, but we can change the email to something non-existent and we can then block it for you, if you wish.
The registered username will remain registered forever, so that it cannot be reused. Without that, somebody could delete an account, then somebody could re-create it with the same name and thus pretend to be them.
To prevent this, usernames are one-time-use only. They cannot be re-created and thus faked, because they cannot be removed. We don’t store any personal data of yours that you cannot change and remove yourself, but the username is not unique enough to be considered to be personal data.