Google does not value user generated content. Update your content yourself. Turn the comments off. If your readers want to get in touch with you, it might be better to use email, install a live chat plugin, install a forum plugin/software, or use social media. 1% of website visitors might leave a blog comment anymore.
However, if you REALLY want to keep comments on and you want to try to filter spam:
I’ve been a spammer for a long time. Captcha is ineffective for the most part with the exception of a couple types. If you can install kcaptcha or hcaptcha that will probably stop the spam. I have not come across any software that can solve kcaptcha without sending it to a paid service and no spammer is going to pay for blog comments. Hcaptcha is close to being solved by software which is available to the general public. So kcaptcha is your best bet.