I recently had to deal with this myself. Rather than openly confronting the individual I attempted to inhibit the activity without a confrontation by blocking the traffic. I enabled content blocking “P2P/File Sharing” which killed all inbound BitTorrent sessions but outbound session(s) remained.
I then configured an outbound firewall rule to block those sessions and there has not been a BitTorrent session since. Some clients will automatically change ports and if this is the case you may have a bit more trouble but this is what I have done and it worked like a champ.
You will need to identify the client device(s) associated with BitTorrent activity and assign them a DHCP reservation to ensure they will receive the same IP, this is for the outbound firewall rule(s). You can identify the elements you need by selecting the BitTorrent sessions listed in the active sessions table.
This worked really well for me and I hope it helps you too. Thanks Almocrin.
(the firewall rule is an example and does not depict the client IP address - your rule will)