Deregister a CSS file that comes with a plugin

You can use wp_dequeue_style function, with a wp_enqueue_script hook with priority higher than WPML’s wp_enqueue_script hook. Put the following code into your functions.php:

function dequeue_wpml_styles(){
    wp_dequeue_style( 'wmpl_style_handle' );
}
add_action( 'wp_enqueue_scripts', 'dequeue_wpml_styles', 20 );

REPLACE ‘wmpl_style_handle’ with the handle WPML registers/enqueues the style.

UPDATE: I have just had a look into WPML and it looks that it doesn’t use wp_enqueue_script to include language-selector.css, But the good news is that there is a constant which you can set to prevent loading of the language selector styles. Just add the line below to your functions.php:

define('ICL_DONT_LOAD_LANGUAGE_SELECTOR_CSS', true);