You can't. The account information is required for audit purposes.
If you contact sales at atlassian dot com, they can delete an account if you've never had a licence, but if you have, then they are required to keep a record of it.
You can (and should, if you don't need it), change the email to a dead email (firstname.lastname@example.org something), then change the password to a random string and simply forget you ever had it...
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