You must find the name of the enqueued stylesheet in your theme and use it to dequeue a style – wp_dequeue_style. URL is unnecessary.
Example:
function remove_unwanted_css() {
wp_dequeue_style( 'font-awesome' );
}
add_action( 'wp_print_styles', 'remove_unwanted_css', 100 );
Or another solution wp_deregister_style/:
add_action( 'wp_enqueue_scripts', 'remove_default_stylesheet', 20 );
function remove_default_stylesheet() {
wp_dequeue_style( 'font-awesome' );
wp_deregister_style( 'font-awesome' );
wp_register_style( 'new-font-awesome', get_stylesheet_directory_uri() . '/new.css', false, '1.0.0' );
wp_enqueue_style( 'new-font-awesome' );
}