We’ve ban people and stories on Netscape every day for doing bad stuff. Sock puppetry, group voting, scraping content, being a splog… the list goes on and on.
One thing that’s worked well is that when we think we can turn someone around, we ban them for a period of time (think weeks to months). We try to explain to these folks how the system works and how they can be good members and I’m shocked to see how often it works. People stop creating multiple accounts, they stop breaking the middleman rule, and they start acting like good citizens.