Remove frontend.css

I’d like to remove the CSS that this plugin injects into my page. I have had partial success with this in my functions.php:

add_action( 'wp_print_styles', 'my_deregister_styles', 100 );
    function my_deregister_styles() {
       if ( !is_admin() ) {
            wp_deregister_style( 'wprssmi_template_styles' );
            wp_deregister_style( 'wprssmi_styles' );
            wp_deregister_style( 'frontend' );
        }
    }

That last line should take care of frontend.css, but it doesn’t. frontend.css is still loaded. How do I get rid of it without altering the plugin files?

http://wordpress.org/plugins/wp-rss-multi-importer/