StumbleUpon is a network specially used by the webmaster to share their web page on the web
1. Simply , Log in to your Stumbleupon account in a New Window.
2. Then click here
3. Then Re-enter your password.
4. Then click on Submit Your Deletion. done
Note :- As per program policy of Stumbleupon, your account will be deleted within 2 week. So if you want to re-activate your account , then Log in again with your default Username and Password.