Restrict payment gateways for Order Form for WooCommerce orders to the Cheque payment gateway. Available since version 1.8.0. https://orderform-woo.webaware.net.au/

orderform-woo-cheque-only.php

<?php
/*
Plugin Name: Order Form for WooCommerce Cheque Only
Plugin URI: https://gist.github.com/webaware/54b8e8a951224c3bef55464632dfd139
Description: Only allow the Cheque payment gateway for order form purchases
Author: WebAware
Author URI: https://orderform-woo.webaware.net.au/
*/

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

/**
* restrict payment gateway for order form purchases
* @param array $gateways
* @return array
*/
add_filter('woocommerce_available_payment_gateways', function($gateways) {
    $cart = WC()->cart->get_cart();

    foreach ($cart as $cart_item) {
        // was this item added from an order form?
        if (!empty($cart_item['orderform_woocommerce']['flag'])) {
            // get cheque payment gateway
            if (!empty($gateways['cheque'])) {
                $cheque_gateway = $gateways['cheque'];
            }
            else {
                $cheque_gateway = new WC_Gateway_Cheque();
            }

            // only allow cheque payment gateway
            $gateways = array('cheque' => $cheque_gateway);
            break;
        }
    }

    return $gateways;
});