Naeem,
The quizzes that you took cannot be cleared because they are the basis of your simulator statistics. The simulator calculates your % based on your answers in the quizzes. So without these, you have no statistics.
Also, the questions that you took will appear again in the future. So if you take the same quiz again and again (for example if you take a quiz with 50 questions from Integration 10 times) then you will see some questions several times. (Exception: If you select "Include only unanswered questions" then you will not see repeat questions.)