Tip on Disabling Buttons

To disable buttons you can use  mcInstanceName.enabled = false; 
This property works on both button symbols and movieclip symbols acting as buttons.

If your button only has it's hit frame defined, or your moviclip serving as a button is  _alpha = 0; , and you wish to enabled another button, which lies underneath use the  _visible property instead;
 mcInstanceName._visible = false;

Also,  you can hide the default button cursor with  mcInstanceName.useHandCursor = false;