A couple of terminology things you need to understand.
An inbound/incoming connection refers to someone initiating a connection to you from the outside. In the case of bittorrent, this is someone connecting to you. An outgoing/outbound connection is you connecting to something else. In the case of bittorrent it could be another peer or the tracker.
The firewall generally allows response packets to an allowed outbound connection even if that port is blocked inbound. This means that even though it is somewhat counterintuitive, to block your own download traffic you need to block outbound connections.
Lastly, blocking bittorrent traffic is extremely difficult. The protocol is made to be as resistant to blocking as possible. Something like gufw isn’t going to be able to block bittorrent effectively unless you use a client that allows you manually put artificial limits on the communication. Alternatively, you could block all outbound traffic. This would definitely stop bittorrent traffic but it would also stop everything else.
If you could explain why you are trying to block bittorrent traffic, maybe we could offer a different solution. Although, as I said earlier, it is extremely difficult to block so maybe not.