Math: Option of changing the setting on quizzes so they can only be taken once.

I teach 6th grade math and I am trying to get my students to slow down, read the problems, write down their work on paper, and approach math with a more deliberate practice.

When they take the Khan Academy quizzes, I can see that they are spending 1-3 minutes racing through the quiz to see if they can get a high score and then they are retaking it up to 7 times in order to improve their score. I would like the option of turning "Off" the retake in the hopes that they slow down.

Thank you in advance for your help in this matter.