What finally worked was a combo of: custom javascript on the signup form, disallowing new users from posting links in posts or signatures (and auto-banning ones who continually made attempts), insta-banning users who filled out non-linkedin links in the "linked-in link" profile field. We still get spam, but now it's maybe one post a week, instead of 10-20 a day, so it's at a level that moderators/user reports can handle it.
Rel=no follow discourages spam due to PR flow, but won't stop it. It still has an influence, however diminished.
Take an hour or two and write a script that selects all users with less than 100 posts and iterates through their posts removing links. The integrity of actual users is maintained while 99% of spam links eliminated. You spend a total of 2 hours, but don't have to deal with the emails any more - which could save many more hours.
No decent spammer cares about whether or not the links are nofollow. Targeting just dofollow links and then having a backlink portfolio to your site that is 99%+ dofollow links looks completely unnatural.
Does this constitute a long-term solution to the problem for active communities? That is, default to rel='nofollow' for an untrusted class of users, which presumably includes all the spammers?
Not at all. I was addressing the fact that when a 3rd party site asks for links to be removed (and are un-willing to pay for 'time' it takes to take links down) and then threaten to turn you into the google police for link spam... you can defend your position and show that you indeed are not participating in link shenanigans.
If you have a relatively small site.. curating content isn't too bad... limiting the tail of articles so that comments can only be added for X days helps with that. Only allowing logged in users to comment (and making login easy via oauth sites like facebook/twitter/g+ etc) I kind of like Disqus, but would rather have something more home-grown.
Then with an auto-reply (or 'link policy' page) inform 3rd party sites that the link-juice (good or bad) is no longer flowing.