What typically happens in a single torrent is that at the beginning upload and download ratios are reasonably correlated, then over time people finish downloading, and some of them leave, but a significant number of seeds remain. After a while there's a period of time where there are many more seeders than downloaders. Anyone who joins the torrent at this late time will be generally get a download rate limited by their download capacity, and anyone who tries to upload to them will only be able to do so at a low rate. Such people will have very out of whack upload/download ratios, but they're downloading from otherwise unutilized upload resources, and hence not being anywhere near the drain on the system that their total upload/download ratio indicates.
A better approach would be to not count it against people when they download from heavily overseeded torrents. Or to just not use total upload/download ratios at all, or if you do only make them advisory and not a source of banning.