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.
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.