Google has published some tips to help site owners, forum owners, and publishers combat auto generated comment spam.
Much of the suggestions are obvious, but the fact that Google is mentioning this suggests to me that spammy comments really does devalue the content, and we know google wants to give users great results. As publishers and site owners we do not want to devalue our sites in any way, and we don't want it to be hurting our rankings.
Google's tips include, as I said, some obvious aspects, such as making sure the forum software is up-to-date and fully patched. Adding a CAPTCHA, using, for example, reCAPTCHA, Secureimage and Jcatptcha. Blocking what it describes as suspicious behaviour is a good move, including for excessive traffic from individual IP ranges. Forum posters coming from nowhere and producing an excessive number of posts are often indicators that something is up. Closing old forum threads that aren't likely to gain up-to-date comments.
It goes on, but, really, there's nothing new here, and it's a shame that some of Google's tools are not made available in some form or another to webmasters and forum owners.