I figured it out! Ultimate member allows you to use a field for user name or email as well as the standard fields user name and email fields. That is what I am using because we want the username to be the same as the email address. So in your code I replaced email with username and it worked:

	public function ban_um( $args ) {
		$email = isset( $args['username'] ) ? trim( $args['username'] ) : '';

		if ( empty( $email ) ) {
			return ;
		}

		if ( ! $this->is_allowed( $email ) ) {
			UM()->form()->add_error( 'username', $this->settings->get_error_message() );
		}
	}