Hi,
You can use ReCaptcha if you want. You didn't ask for it specifically, so I didn't mention it.
We propose our own ReCaptcha plugin:
www.hikashop.com/marketplace/product/223...-with-recaptcha.html
You can use it instead of Joomla's. It works with the Joomla registration page, the HikaShop registration page, the Joomla contact form and HikaShop's contact form.
We also developed more recently an integration plugin between HikaShop and Joomla's captcha system:
www.hikashop.com/marketplace/product/281...-joomla-captcha.html
This will allow you to use the same captcha system as the one you configured in Joomla and it can work for both HikaShop's registration page and HikaShop's contact form.
Now, why I didn't mention them in the first place ?
That's because nowadays, Recaptcha and other captcha systems can be easily circumvented by spam bots. OSpam-a-not works better, from what we could find. And that's why we're using it instead of ReCaptcha on our own website.