Overriding the X-Forwarded-For header in haproxy?

Tear it off the request header, before HAProxy adds its own:

reqidel ^X-Forwarded-For:.*

The risk to this change is that you'll lose your information about the "real" client IP address - your logs will then be showing the IP of the proxy server that the client is using. Sounds like you're ok with that!

As an aside, see this question for some interesting info on confusion over the append order of the X-Forwarded-For header.