It’s not possible to delete your account. This is what we can find in their FAQ:
If you have any questions about our privacy statement or our use of your information, or if you need help changing or deleting your Starbucks.com account you can e-mail us at [email protected]
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.