On their site, after searching a bit, you can read it’s impossible to delete an account. (Source: http://turbotax.intuit.com/support/iq/Multiple-Accounts/Problems-associated-with-multiple-TurboTax-accounts/GEN21940.html ).
So, with all these possibly filled in personal (and financial) data, that doesn’t sound too good. Advice: delete and anonymise as much as possible from your account page (log in here: https://qtwu1.qprd.turbotaxonline.intuit.com/turbotax/ttonline.htm and at least delete forms, if necessary; on your right you can change personal data). If you’re still no happy customer, let them know and request deletion anyway: https://turbotax.intuit.com/support/contact/index.jsp
URLS:
http://www.turbotax.com/
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.