Hi,
You can clear your information from the account using the "Clear and Start over" feature. Unfortunately there is no option to delete your account. Once you've paid or registered, we're required to store copies of your return. Just let the account go dormant.
For further information you can refer to the following link:
https://ttlc.intuit.com/questions/1899886-can-i-delete-my-tax-return-in-turbotax-online