I tought a little bit about it.

I think admin (nanotube) should be able to remove ratings that he clearly see is malicious. For instance a user that neg-rates many other uses without having done trades with them, and get a lot of complaints about his actions.

If there's doubt - then a neg rating should not be removed.

Or it could be a decision made of the most trusted users on bitcoin-otc, for instance nanotube, sturles and more.

I've learned that Nanotube refuses to remove any ratings, something that I can understand, but if it gets to a point where the system is abused too much, it might lose it's value, and then action is required.