can't turn off eth0

I can’t shut down eth0
the /etc/network/interfaces file has the setup lines for eth0 commented out, as shipped
ifdown eth0 gives me : interface eth0 not configured
ifconfig eth0 down gives me no response
and in both cases the port stays connected.
Any ideas on how to accomplish turning it off?

edit: seems the ethernet port does disconnect with ifdown but only momentarily, something else is bringing it back up. How do I find out what it is?

systemd maybe ?