Try this link on WikiAnswers. It's the same question you're asking:
Wiki.answer.com - How do you stop SpySweeper from asking for your Online Backup password after you canceled your Online Backup account? (http://wiki.answers.com/Q/How_do_you_stop_Spy_Sweeper_from_asking_for_your_Online_Backup_password_after_you_canceled_your_Online_Backup_account)
Apparently, you have to log into
WebRootBackup Account Login (https://myaccount.webrootbackup.com/login.aspx)
and cancel it from there.
If that doesn't work, you can always reinstall Windows from scratch :mad: (just kidding)
Thanks for your response. However, I had already checked the WikiAnswers link and the guy there only got one response, which did not solve the problem. I also tried to the link you mentioned to log into the backup account, and it just said the account had been canceled, so no help there either. Still stumped.