If you're using Linux you can use iptables to block any outbound packets that's destination IP isn't your VPN IP once OVPN is up; obviously on all interfaces ex

If you're using Linux you can use iptables to block any outbound packets that's destination IP isn't your VPN IP once OVPN is up; obviously on all interfaces except loopback and your OVPN interface. And remove it again once your drop your OVPN connection. You should also make sure that your routing table has an entry for your VPN IP (via your LAN interface / old default gateway), one singular default route that uses your OVPN interface and VPN internal remote address, and lastly that all cached routes are dropped on VPN connection. Not all OVPN configurations will do this correctly, it depends on your distro / configuration.