Contact form is vulnerable to spam bot. Captcha does not help in this case.
Fix here – https://github.com/PrestaShop/PrestaShop/pull/8168/files
Useful information on how to build an anti spam bot contact form without captcha
http://www.nfriedly.com/techblog/2009/11/how-to-build-a-spam-free-contact-forms-without-captchas/
Update:
Doesn’t work well against some automated spammers. Change the friendly url of the contact-us to something else and it stopped the spamming completely.
https://www.prestashop.com/forums/topic/442633-add-recaptcha-to-prestashop-version-16014-for-free/?page=5
https://www.prestashop.com/forums/topic/673164-how-do-i-integrate-recaptcha-v2-into-prestashop/?tab=comments#comment-2697948