I just figured it out!

The subscription can be managed on https://www.classy.org/.  You should have an account from when you signed up.  You will see Khan Academy (or possibly labs) listed in your subscriptions.  

Choose edit subscription (or similar) and there is a red button near the bottom to cancel.

The place in settings (find by clicking your name at the top of the page) they are referring to is a 3rd Khan Labs tab (that isn't all that noticeable)  where you can choose who can use the ai.  I might have followed a link from there to find Classy but don't see one there now that I've cancelled.

Good Luck!