Make sure you are logged in, then use the direct below link to delete your account.
For more info: http://help.stumbleupon.com/customer/portal/articles/665202-account-settings#article17
URLS:
http://www.stumbleupon.com
https://www.stumbleupon.com/settings/delete-account
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.