"The buck stops here", used to be the amusing sign on the desk or wall of the topmost boss.
In recent decades, sadly, it has become common practice for those bosses to use scapegoats. But it doesn't really change the fact that ultimately, you are responsible for the actions of everyone you hire or manage. That responsibility is the job. Anyone paying a spammer to spam, knowingly or not, is responsible for the spam they paid for. Even an honest mistake is a sign that the management was insufficient, not providing enough guidance or monitoring.
Again, wrestling this back to Jon's original post, this applies directly to user-generated content. You should always view UGC as a method of delegation - a way of enabling many hands to make light work - but ultimately, you have to provide enough guidance and management to make it work. Any failing of UGC is your responsibility.
All those years ago, working with dooyoo.co.uk, we had a massive community (10s of thousands) of members who were all submitting content for small payments. Managing and policing the UGC was a very high priority, and when it failed, it could cost us hugely. Like the day we lost a six figure a month content deal to provide consumer reviews to a major UK ISP because the opinion on the front page, just submitted moments before, was full of expletives. That was our responsibility. As I mentioned elsewhere, we found a way to better prevent these incidents, but the largest part was in guiding and educating our community. Making them understand better what we wanted, and why it was in their interests.
Here at Cre8asite we obviously have similar issues. A forum is effectively just a huge comment area with no blog post at the top. Years ago when we faced the first concerns about 'bad neighbourhoods' we decided to implement our own safeguards, with all links going through a redirect that was itself protected and blocked by the robots.txt, and links obfuscated, and generally ensuring that no link counted for anything in search. Any link here, be it dropped in a post, or in a sig, or elsewhere, that did not go to the cre8asite domain could ever be indexed or crawled. If you post a link here, it is for the benefit of the reader(s) only.
Still, Moderators remove the spam. Why do so when it can't have any SEO effect at all anyway? Because of the user effect. A forum full of spammy link drops is a poor user experience that attracts nobody except other spammers.
And that, Jon, is the final answer. Yes, bad neighbourhoods are not just about SEO. Comments full of spam or link drops just plain detract from the user experience and can completely kill your site.