Example of some custom validation rules for woocommerce checkout page. Forcing names to match account holder, custom rules based on shipping classes and shipping locations.

form-checkout.php

<?php

## add this to /wp-content/[your-theme]/woocommerce/checkout ##

/**
 * Checkout Form
 *
 * This template can be overridden by copying it to yourtheme/woocommerce/checkout/form-checkout.php.
 *
 * HOWEVER, on occasion WooCommerce will need to update template files and you
 * (the theme developer) will need to copy the new files to your theme to
 * maintain compatibility. We try to do this as little as possible, but it does
 * happen. When this occurs the version of the template file will be bumped and
 * the readme will list any important changes.
 *
 * @see https://docs.woocommerce.com/document/template-structure/
 * @package WooCommerce/Templates
 * @version 3.5.0
 */

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

//get list of products in the cart
global $woocommerce;
$items = $woocommerce->cart->get_cart();

//get logged in user name
$current_user = wp_get_current_user();
?>
<script>
    //set vars to hold user name
    var user_fname = '<?php echo $current_user->first_name; ?>';
    var user_lname = '<?php echo $current_user->last_name; ?>';
    
    //push all shipping classes into an array
    var shipping_classes = [];
    <?php foreach( $items as $item) { ?>
        shipping_classes.push("<?php echo $item['data']->get_shipping_class(); ?>");
    <?php } ?>
</script>

<!-- container for custom error messages -->
<div id="customAlert" class="woocommerce-error alert alert_error" role="alert" style="display:none;">
    <div class="alert_icon"><i class="icon-alert"></i></div>
    <div class="alert_wrapper"></div>
</div>

<?php

do_action( 'woocommerce_before_checkout_form', $checkout );

// If checkout registration is disabled and not logged in, the user cannot checkout.
if ( ! $checkout->is_registration_enabled() && $checkout->is_registration_required() && ! is_user_logged_in() ) {
	echo esc_html( apply_filters( 'woocommerce_checkout_must_be_logged_in_message', __( 'You must be logged in to checkout.', 'woocommerce' ) ) );
	return;
}

?>

<form name="checkout" method="post" class="checkout woocommerce-checkout" action="<?php echo esc_url( wc_get_checkout_url() ); ?>" enctype="multipart/form-data">

	<?php if ( $checkout->get_checkout_fields() ) : ?>

		<?php do_action( 'woocommerce_checkout_before_customer_details' ); ?>

		<div class="col2-set" id="customer_details">
			<div class="col-1">
				<?php do_action( 'woocommerce_checkout_billing' ); ?>
			</div>

			<div class="col-2">
				<?php do_action( 'woocommerce_checkout_shipping' ); ?>
			</div>
		</div>

		<?php do_action( 'woocommerce_checkout_after_customer_details' ); ?>

	<?php endif; ?>
	
	<?php do_action( 'woocommerce_checkout_before_order_review_heading' ); ?>
	
	<h3 id="order_review_heading"><?php esc_html_e( 'Your order', 'woocommerce' ); ?></h3>
	
	<?php do_action( 'woocommerce_checkout_before_order_review' ); ?>

	<div id="order_review" class="woocommerce-checkout-review-order">
		<?php do_action( 'woocommerce_checkout_order_review' ); ?>
	</div>

	<?php do_action( 'woocommerce_checkout_after_order_review' ); ?>

</form>

<?php do_action( 'woocommerce_after_checkout_form', $checkout ); ?>

<script>
    
    //create rules to match city or state, as well as the prohibited shipping class for that location
    //add as many as you like. This currently only works for city/state vs shipping class
    var rules = [
        {'param':'State','value':'WA','Class':'shipping-class-slug'},
        {'param':'City','value':'San Francisco','Class':'shipping-class-slug'}
    ];
    
    //when any field changes
    jQuery('input').blur( function(){
        //reset variables
        var passing = true;
        var box_checked = false;
        
        //is the shipping box checked?
        if( jQuery('#ship-to-different-address-checkbox').prop("checked") ) {
            box_checked = true;
        }
        
        //process each rule
        rules.forEach( function(rule){
            //location to check
            var check1 = rule['value'];
            //console.log(check1);
            var check2 = '';
            
            //get the entered location
            if( rule['param'] == 'State' ) {
                if( box_checked ) {
                    check2 = jQuery('#shipping_state').val();   
                    console.log(check2);
                } else {
                    check2 = jQuery('#billing_state').val();
                    console.log(check2);
                }
            } else if ( rule['param'] == 'City' ) {
                if( box_checked ) {
                   check2 = jQuery('#shipping_city').val();
                    console.log(check2);
                } else {
                   check2 = jQuery('#billing_city').val();
                    console.log(check2);
                }
            }
            
            //fail if the rule is matched
            if( check1 == check2 && shipping_classes.indexOf(rule['Class']) !== -1 ){
                passing = false;
                //console.log(rule['Class']);
            }
        });   
        
        if( !passing ){
            //disable the shipping button if rule matched
            jQuery('#place_order').prop('disabled', true);
            
            //display an error message and scroll up to it.
            jQuery('#customAlert').show();
            jQuery('#customAlert .alert_wrapper').html('We are unable to ship one more items due to city/state laws where you are. Please review your purchase and try again.').focus();
            jQuery([document.documentElement, document.body]).animate({
                scrollTop: jQuery("#customAlert").offset().top - 200
            }, 1000);
        } else {
            //turn the shipping button back on if no rules matched
            jQuery('#place_order').prop('disabled', false);
            
            //hide the error block
            jQuery('#customAlert').hide();
        }
    });
    
    //force billing & shipping names to match account holder name
    jQuery(document).ready(function() {
        jQuery('#billing_first_name').val(user_fname).attr('disabled', true);
        jQuery('#shipping_first_name').val(user_fname).attr('disabled', true);
        jQuery('#billing_last_name').val(user_lname).attr('disabled', true);
        jQuery('#shipping_last_name').val(user_lname).attr('disabled', true);
    });
    
</script>