Actually, it does work.
Plus, there are two problems with the way you suggest. First, this means that on the affiliate sign-up page, the lowest affiliate percentage is shown. This in turn means that I would have to adjust each affiliates percentage manually. Since ejunkie does not advise me when a new affiliate signs up, this would mean I would have to check for new affiliates, manually, every day. Not.
Second, the whole point of this exercise is to track a particular ad, which I have tied to a particular affiliate since there is no built-in way to do this in ejunkie. So banning the account defeats the entire purpose.
Thanks for the feedback, but the negative percentage is workin just fine.