StumbleUpon used to list its account deletion links in its help center. Since the revamp of its user interface, the help center was one of many features to go missing. But it’s still possible to delete your StumbleUpon account.
To delete your StumbleUpon account, visit the account deletion page at: https://www.stumbleupon.com/settings/delete-account
StumbleUpon will ask you to enter your password and then push the orange submit button to confirm deletion.
The process isn’t instant — just like deleting Facebook and Twitter accounts, and you can always cancel a deletion within a two week period of starting it. To cancel, just go back to the same deletion page and there will be a cancel button there instead.
During the two week deletion process you can still use the StumbleUpon account as normal. Your account won’t be automatically re-enabled though. Just be ready for it to vanish when it hits the 14 day mark.