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.
Note that this will not delete the student's account. It will simply un-assign this quiz from them.