To delete your CK-12 account, just follow these steps:
1) Click your name in the upper right corner of the site.
2) Click Settings.
3) Scroll to the bottom, and choose to deactivate or delete your account.
Deactivating an account leaves the account in tact, but renders it dormant. You would need to reactivate your account to use it.
Deleting an account anonymizes your data and makes the account completely inaccessible. There is no way to reactivate a deleted account, or to retrieve any data, classes, or artifacts from it. Note that there is a 30 day delay in deleting accounts, in case you change your mind.