I have a little something I thought would be nice. Just a thought. Have a count, if someone rates someone 0 give them a -1, if someone rates someone 5 give them a +1. For 2,3,4 you can calibrate accordingly. So that people who do mediocre postings get to stay as well. After a threshold has been crossed, write a trigger to ban them. If its a first time let them come back after 2 days. If its 2nd after 3 days. If its third, bye bye.
This way, once you implement it, you don't have to worry about the human emotions coming to stop you from doing the right thing.