The ads are provided through Yahoo's ad service. eBay would have to ban those sites through Yahoo's ad service's process. There may be a limit to how many sites they can ban. The best thing to do, when you email them, is to give very specific example, like you did here - do either a screen shot and attach that, or copy and paste entire ads. They do not know what their users see specifically, so the more information you give them, the better chance they can do something about it.