Closing down public interface

I set up haproxy and a couple of servers behind it, all with private networking enabled. I want to shut down public access to the servers directly, so I configured haproxy to use the private IPs of the servers, which works.

What doesn’t work is shutting down the public network interface on the servers. If I do “ifconfig eth0 down”, the server stops responding to the haproxy.

Is this approach wrong? If it is, what’s the right way to block bypassing of my front end proxy?