A simple, custom SPAM filter for WooCommerce registration forms. This just prevents registration form submissions unless the custom field has the expected value in it.

functions.php

<?php 

include 'SimpleWooRegistrationSpamFilter.php';

\PhilKurth\SimpleWooRegistrationSpamFilter::init();

SimpleWooRegistrationSpamFilter.php

<?php

namespace PhilKurth;

use WP_Error;

/**
 * Class adds a simple math equation to the registration form in order to help combat SPAM submissions.
 */
class SimpleWooRegistrationSpamFilter {

	private static $question = 'One plus three equals?';
	private static $answer = 4;
	private static $field_name = 'reg_test_human';

	public static function init() {
		// Add the field to the form.
		add_action( 'woocommerce_register_form', [ __CLASS__, '_render_field' ] );
		// Validate the input.
		add_filter( 'woocommerce_registration_errors', [ __CLASS__, '_validate_field' ], 10, 3 );
	}

	public static function _render_field() {
		?>
		<p class="woocommerce-form-row woocommerce-form-row--wide form-row form-row-wide">
			<label for="reg_test_human"><?= self::$question ?>&nbsp;<span class="required">*</span></label>
			<input type="number"
			       required
			       class="woocommerce-Input woocommerce-Input--text input-text"
			       name="<?= self::$field_name ?>"
			       id="reg_test_human"/>
		</p>
		<?php
	}

	public static function _validate_field( WP_Error $errors, $username, $email ) {
		// If the field isn't in the request, don't validate. This ensures new customers being created during checkout
		// aren't affected by the field.
		if ( ! isset( $_REQUEST[ self::$field_name ] ) ) {
			return $errors;
		}

		if ( empty( $_REQUEST[ self::$field_name ] ) ) {
			$errors->add( 'registration-error-invalid-math-equation', 'The answer to the math equation is missing — try again.' );

		} elseif ( $_REQUEST[ self::$field_name ] != self::$answer ) {
			$errors->add( 'registration-error-invalid-math-equation', 'The answer to the math equation is incorrect — try again.' );
		}

		return $errors;
	}

}