disable trashability for certain pages

Add this to your functions.php in your theme folder:

function restrict_post_deletion($post_ID){
    $user = get_current_user_id();
    $restricted_users = array(21,25,54,2,19);
    $restricted_pages = array(2,21,52,64);
    if(in_array($user, $restricted_users) && in_array($post_ID, $restricted_pages)){
        echo "You are not authorized to delete this page.";
        exit;
    }
}
add_action('wp_trash_post', 'restrict_post_deletion', 10, 1);
add_action('wp_delete_post', 'restrict_post_deletion', 10, 1);

Taken from here and updated to the latest WP version.