One way to do this would be to simply hide the upload/insert media button: and, then add featured image support for your themes posts, so a user can still attach images to the post.
Hide upload/insert media button for your theme in: functions.php:
if($hook != 'post.php' AND $hook != 'post-new.php')
return; // if not in the edit or new admin pages, for Posts or Pages, then do nothing.
echo '<style type="text/css" media="screen">
Add featured image support to your themes posts in: functions.php
add_theme_support('post-thumbnails', array( 'post' ));
I didn't realize you were referring to attaching multiple images to a post. You could just use the css I gave above to hide the button for inserting images into a post: and, then use one of the plugins that come up in a google search for attaching multiple featured images to a post: