Recently I was watching a Google Hangout about SEO (Search Engine Optimization) when . . .
A participant asked this question:
"There are occasions when I see comments on my blog that contribute nothing of real substance such as "I am so glad I found your blog." And if you check out the person, they leave the "exact same comment on other websites." These kinds of comments are more like spam. Although even worse are comments that leave links to the commenter's website. I've always wondered if poor quality comments affect my website ranking."
The answer from Google was very clear.
Google considers "comments" from other people on your blog the same as your content. This is because you have control whether or not the comments appear on your blog. Therefore all comments are considered your published content.
Comments with poor spelling, bad grammar, lacking authority, or disreputable quality will be associated with your site. The bottom line is "it is on your website." Google recommends that you "take action" to maintain the quality of all content on your website, even if there are no spammy links.
With this insight, as the publisher of your blog, you should edit comments for grammar at a minimum, correct punctuation or spelling, remove all links that are unrelated to the post content, and actually delete comments that do not add content to your post.
Harriete Estel Berman