How to remove your Wolfram Alpha account
Seems that they are working on proper account removal!
From their FAQ:
“For the immediate future, send a message to firstname.lastname@example.org, and your Wolfram ID will be removed from our system manually. Soon there will be an option for you to do this yourself.”
Helpful Trick: Removing online data
Helpful Trick: Remove third-party logins
If you signed up for Wolfram Alpha using a third-party login such as Facebook or Twitter, you probably want to revoke all access from Wolfram Alpha. Click here to find out how.