You could shoot the hostage and add rel='nofollow' to all outbound links. You should be able to programmatically do this.
Then with an auto-reply (or 'link policy' page) inform 3rd party sites that the link-juice (good or bad) is no longer flowing.
Then with an auto-reply (or 'link policy' page) inform 3rd party sites that the link-juice (good or bad) is no longer flowing.
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.
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.