wp_authenticate_spam_check( WP_User|WP_Error|null $user )

function wp_authenticate_spam_check( $user ) {
	if ( $user instanceof WP_User && is_multisite() ) {
		/**
		 * Filters whether the user has been marked as a spammer.
		 *
		 * @since WP-3.7.0
		 *
		 * @param bool    $spammed Whether the user is considered a spammer.
		 * @param WP_User $user    User to check against.
		 */
		$spammed = apply_filters( 'check_is_user_spammed', is_user_spammy( $user ), $user );

		if ( $spammed )
			return new WP_Error( 'spammer_account', __( '<strong>ERROR</strong>: Your account has been marked as a spammer.' ) );
	}
	return $user;
}