look for the file post.php then look for this code:
<?php if ( current_theme_supports( 'get-the-image' ) ) get_the_image( array( 'meta_key' => 'Thumbnail', 'size' => 'single-thumbnail', 'link_to_post' => false, 'image_class' => 'featured', 'attachment' => false, 'width' => 470, 'height' => 260 ) ); ?>
simply delete that part and save. the featured image on the posts will disappear but the featured posts on archives will still be there. it’s what i did with my site