This isn't a PrestaShop issue. Spam is a global problem for all websites with forms. This can be fixed in PrestaShop, but again, it's not just a PrestaShop issue. A few years ago, I created a module that protects contact forms on multiple levels, not just captcha, but also filters for words, email addresses, domains, etc., which is more than enough to stop spam. I have many satisfied customers who use this module and don't need external services to secure their forms.