Any chance of blocking these?

[Ljósnet]

Hello, I have a small problem with few MS Exchange servers who are
infected with some kind of virus/trojan. They are sending mail through
my gateway which is fine, but I need to be able to stop them from
sending mail which dont have any valid address in from=< > field until
I get the system administrator to clean their server.

In the maillog it looks like this:

sendmail[12542]: n56Ki9EY012542: from=<>, size=23860, class=0,
nrcpts=1, msgid=<fiplj8nhX00000e8f at domain.com>, proto=ESMTP,
daemon=MTA, relay=[10.101.45.50]

Is it possible to somehow stop servers/clients from being able to send
through my gateway when there is no valid from= address?

Thanks!