If you want to delete the return you've created, go to Tools at the top of the screen, Return Manager, and then delete the return. If you only have one return created, you will first need to go there and create a new return (it'll be blank) and then go back and delete the return you wanted - you just can't delete an active return.
What you can do is remove all personal information from any returns in the account, and while you cannot delete the account itself, if you do not want to receive reminders or notifications, you should be able to change the email address associated with it to a non-existent one. Note that if you do this, you will not be able to recover the account.
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