Yes, comment spam (for the purpose of getting backlinks to your site) is against Google’s Webmaster Guidelines, as stated by Google’s Matt Cutts:
“If your primary link building strategy is to leave comments all over the web to the degree that you know you’ve got a huge fraction of your link portfolio in comments and no real people linking to you, then at some point you know that can be considered a link scheme. At a very high level we reserve the right to take action on any sort of deceptive or manipulative link schemes that we consider to be distorting our rankings. But, if you’re just doing regular organic comments, and you’re not doing it as, you know, okay I have to leave this many comments a day every single day because that’s what I’m doing to build links to my site, you should be completely fine.” source
If you allow people to place comments on your website without moderation (moderation seems to be used as a quality signal), you are at risk of getting penalized. And don’t expect spammers to stop doing this; they will always spam, because that’s their job (decent black-haters will do this on tiered backlinks).
Google also states that:
“if your site has too much user-generated spam on it, that can affect our assessment of the site, which may eventually result in us taking manual action on the whole site. Some examples of spammy user-generated content include […] comment spam on blogs” source