Docker with a gluetun container. This container's only role is to connect to my VPN provider. Any traffic I want through the VPN is set to use that container as its network. If the VPN goes down, gluetun loses connection, any container attached to gluetun can no longer access the internet.
I use a qbittorent docker container for my torrent client but you can attach whatever client you're comfortable with, as well as any other container you prefer be on a VPN.
While qbittorent has a built in kill switch, this works for everything. No VPN means no connection, period.