See WordPress SEO by Yoast: Hide Meta boxes in posts for non-admins
add_action('add_meta_boxes', 'yoast_is_toast', 99);
function yoast_is_toast(){
//capability of 'manage_plugins' equals admin, therefore if NOT administrator
//hide the meta box from all other roles on the following 'post_type'
//such as post, page, custom_post_type, etc
if (!current_user_can('activate_plugins')) {
remove_meta_box('wpseo_meta', 'post_type', 'normal');
}
}