This would be bad enough if it was centralised and part of helping the moderators distribute ingame karma or whatever fFrogboy mentioned earlier, but it seems a third party site dedicated to such things will just turn into mud slinging at people for no real raeson. Sure in the happy go perfect world this might be useful to a degree, but this is the internet with anonyminity. Not to mention what use is such a site, who is going to alt tab to the banlist in every custom game they play and then bitch out anyone on the list.
This, if it did happen, would ruin the game experience for this person and they may not even know what they have been accused of. The whole idea while may have had good intentions seems ultimately flawed and will only serve to either disrupt the community, or just become a source of people bitching at each other.
If you play someone you don't like playing with personally, keep a note yourself, or wait for a feature that lets you track these people, there is no reason to share your greivences with another player with everyone on the planet.