https://yottasrc.com/wiki/article?t=how-to-block-outgoing-traffic-to-private-networks-using-ufw-on-your-server

Stop all incoming and outgoing traffic then allow only the VPN remote port number out to the Internet.

Remember to allow inbound connections from your local network to the management ports if you need them.

Do the same on your network firewall, block all outbound traffic from the torrent box IP address then allow only the remote vpn port out.

It’s probably best to handle that at the firewall, host based, external, or ideally both. The only traffic allowed outbound from the torrent box should be the VPN connection. Then it doesn’t matter if routing or interface binding is set up wrong.