Reset Only Once for 1 user

A large som of students can make the quiz, setting is “only-once.”

One of the students made a mistake by starting the quiz.
How can I make her make the quiz again?
I already tried to :
1. delete the statistics,
2. delete the cookies,
3. deactivate the checkbox only once,
4. reset the user identification button.

But all this wasn’t working.
On which table of where is the checking so I can do it manually?