Keep your site clean of spammers, especially in places where spammers can add content to your site, is important. Those who fill up the site's comments area with irrelevant content can hurt a site from being trusted by both Google and users. Yes, I know the comments here are pretty wild but they are mostly on topic - I do try to keep the pure spam stuff off the site.
Google's John Mueller said it is not just important to block Google from seeing these spammy comments but also your users shouldn't see it. So don't just block it from Google, block it from users. Even more so, John recommends that when you block spammers, make sure they know it, so they don't keep trying.
John wrote this in a Google Webmaster Help thread, "they [spammers] should notice that they're being blocked, otherwise they'll continue to ruin your website."
Here is the full quote:
A "noindex, nofollow" is sufficient for us on spammy pages like this, though it'll take quite some time to be dropped, and *your* users might see it within your site if you don't really remove it there. That said, I'd recommend aggressively battling these UGC spammers to encourage them to move on. Find ways to restrict signups, find ways to vet the content that's uploaded, take aggressive steps to get rid of all of these spammers. They should notice that they're being blocked, otherwise they'll continue to ruin your website.
Forum discussion at Google Webmaster Help.
