Can we act on this question?
The question basically asks how to set up a firewall on Linux that blocks non VPN connections.
Would a rewording of the question be useful in order to re-open it?
Can we move it to Server Fault / Super User and re-open it?
I also note that a lot of questions on SE explicitly asking "how to do this with openvpn" stays open. See for example this question asking "how can I still change the DNS [using openvpn]".
Only the question in subject got closed.