Expanding on Shines good find that its an iframe, i came up with this:
If you can get a HTML element in next to the iframe (so that its a sibling to the iframe) you could make the parent and the iframe position relative and this new element (we can call the new element blockie) position absolute. now make sure that the z-index of blockie is higher than the z-index of the iframe, then use css attributes "top" and "right" to position blockie over the logo
Change your theme's comments.php to follow this example:
<?php comment_form(); ?>
This will block the button. You may want to change "blockie" background colour to match the background of the comment box. I admit its not the nicest solution, but it does work!
Good luck Travis!