Our OE still uses SysV init, not systemd.
Quickest is to go into /etc/rc3.d and remove the OpenVPN symlink.
Thank you so much, WanWizard, for your support.
I've proceeded to remove the symbolic link on /etc/rc3.d, as I need Openvpn to work on demand, but I've noticed that this service (based on Expressvpn) starts randomly at times and I can't explain why.