To remove a student from a quiz that you've already set, first click on the quiz. Then find the student you want to remove, for example, student Isaac Newton in the example below:

Next, select the student's name. In the box that appears, click "Remove <student name> from this quiz". Here's what this looks like:

A confirmation pop-up will make sure you want to remove this student. It looks like this:

Click "Yes" if you'd like to proceed. The student will be removed, and so will any of their answers.
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Note that this will not delete the student's account. It will simply un-assign this quiz from them.