Sell only in specific states/regions/provinces

functions.php

<?php
/**
 * Sell only in California
 */
function wc_sell_only_states( $states ) {
	$states['US'] = array(
		'CA' => __( 'California', 'woocommerce' ),
	);

	return $states;
}
add_filter( 'woocommerce_states', 'wc_sell_only_states' );