I did not agree to anything. When I open the site they just start serving videos to me (even autoplay is activated by default). If they don’t want me to watch their videos without ads they should stop serving them to me (ie, put them behind a paywall)

If qBittorrent is not complaining about file errors you are in fact still seeding the original file. Especially on Linux file systems a file keeps being referenced as long as at least one application is still accessing it, regardless if you delete, rename or alter it. Once you close qBittorrent the ‘old’ file will be dropped though.

And at that point it won’t be seeded anymore as it does not match the checksums that are stored in the .torrent file or were retrieved via a magnet link. If the client is not broken it should not be possible to seed corrupted or altered files.