How to block torrent upload completely?

Transmission

As an alternate solution to your issue which is effectively stopping uploads, I'd suggest switching to one of the Linux' native torrent applications, like Transmission.

It has an inbuilt feature which allows you to stop uploading completely.

This is done by limiting globally the permitted upload bandwidth to 0 Kbps.

Deluge

You can set either a global option of 0 = maximum upload speed or individually.

Set the options for your magnet either when it opens or after you have added the torrent to your list.

I select my download and upload settings for each individual torrent, so I have left the global option on -1 = unlimited and customize the individual torrents.