If you're wanting to disable mouse interaction, this should work.
Put a DIV over the top of your iframe, like so (replace top, left, width and height values with your own):
<div id="blank" style="display:none; position:absolute; top:100px; left:100px; width:600px; height:400px;">
and when you want to disable the iframe, do this in javascript:
document.getElementById("blank").style.display="block";