How you remove the incorrect feedback movieclip depends on how you make it appear to begin with. Chances are whatever you do will get done with the code that executes with the Back button.
I do not know how to answer your last wondering. Not becase I can't figure out a solution, but because I do not understand how the user can cheat and find the correct answer and no read the review. What you describe almost seems to say that you do not want them to be able to go Back and answer a question again... if that is the case then get rid of the Back button.