but you are already uploading while you are still downloading. and that can't be turned off. if seeding scares someone, then uploading should scare them too. so they are right, because they are required to upload.
For public trackers maybe.
Almost every client let you set uploading limit, which you can set at 0. The only thing that generate upload bp usage that cannot be deactivated would be protocol stuff (but you can deactivate part of bt like using the DHT).
Now, as a seeder, you may still be interested in those clients being able to download and reach whatever information you are seeding.
In the same vein, as a seeder, you may just not serve those clients. That's kind of the beauty of it. I understand that there may be some old school/cultural "code of conduct" but really this is not a problem with a behavioral but instead with a technical solution that happens to be already built-in.
well, yes and no. legal issues aside (think about using bittorrent only for legal stuff), the whole point of bittorrent is that it works best if everyone uploads.
actually, allowing clients to disable uploading is almost an acknowledgement that illegal uses should be supported, because there are few reasons why legal uses should need to disable uploading.
and as an uploader i also don't want others not to upload. so while disabling upload is technically possible, it is also reasonable and not unlikely that connections from such clients could be rejected.