We're seeing a lot of unwanted attempts to relay traffic through our
site by Orange.fr, and we've reported this to their Abuse contact as
well as their upstream provider (rain.fr):

Jul 11 11:30:37 mail mimedefang.pl[31610]: relay: bad tld orange.fr
Jul 11 11:30:37 mail mimedefang.pl[31610]: filter_relay rejected host (smtp-wifi.orange.fr)
Jul 11 11:30:37 mail sendmail[32044]: l6BHUb3j032044: Milter: connect: host=smtp-wifi.orange.fr, addr=, rejecting commands

No joy.

We'd like to take escalatory measures now. What is a good RBL site (or
as appropriate) to get them listed on until they start "playing well
with others"?

Would the FAQ's "Reporting Spam" section be a good place to mention the
various sites that you can rat out offenders?