Thanks for the suggestion! We will consider it for the future versions of the plugin!
However, please note that there are no perfect captcha solutions. They all load additional scripts and slow down your website. Also, they don’t give you 100% protection from SPAM.
Therefore, our recommendation is to completely disable submission of reviews on product pages and collect reviews by invitation emails only 🙂 There is almost no chance that a real customer will come to your website to write a review. It will most likely be a bot that wants to post some SPAM on your product pages. So, is it really worth it to allow posting reviews on product pages and deal with SPAM and captchas when nobody will write any reviews there anyway?