Although I disagree with what that person is doing, stopping them is a little difficult. Stopping them would, reasonably entail, stopping everyone who sells fansubs. eBay is filled with people pawning off fansubs without a creditting the fansubbers (I've seen some auctions go as high as $40/per disc).
They're a problem, but there isn't much fansubbers can do about it. However, if they're selling licensed anime, corporate america would be more than happy to deal with them.
