Hi Mary,

Other than using a lockdown browser that prevents users from leaving the current application, there is not an easy way to disable this cheating strategy. Unfortunately, I'm also afraid that no matter the security measures, there will always be those who try to circumvent the system.

A few suggestions for encouraging students not to cheat: 

• Emphasize the importance of the practice, and the benefits it will have on their SAT score
• Don't assign point values for their practice. Many students will cheat simply to earn the points they need in class. They may be less likely to cheat if they're doing it for their own benefit, rather than for the points.
• Require students to periodically do paper quizzes/exams. This will allow you to see how much they have actually learned, and whether or not they can actually do the problem without using this cheating strategy.

Hope this helps! Please let me know if you have any additional questions!

There was a claim 4 1/2 years ago that this was fixed.

https://engineering.khanacademy.org/posts/no-cheating-allowed.htm

Maybe you can report it as a bug.

https://khanacademy.zendesk.com/hc/en-us/requests/new

New update could have broken it, or there was a hole that wasn't tested.