How do I make all buttons on stage unclickable, while a certain movie clip is visible?

One way would be to make the movieclip clickable.  That would make it block the other objects behind it...

     mc.onRelease = function(){ }

     mc.useHandCursor = false; // to hide the hand cursor

Another way would be to disable the buttons while the movieclip is present...

     btn.enabled = false;

Another way would be to build conditional code into the button clicking code to not work if the movieclip is visible.

     btn.onRelease = function(){

          if(!mc._visible){

                // okay to do stuff

          }

     }