I've installed nfilter. Its a bit dated and quirky but I've been
running it without a glitch for a week. I've set it up so that any
crossposted articles never show. Since any header field can be used,
there are a lot of possibilities.

I'm running XP Pro with Thunderbird newsreader. There is a faulty
version of nfilter out there that won't install on XP - here's the one
I've been using (I think!):
http://www.netaxs.com/home/v/nfilter/download.html

Also, here's a link to the nfilter FAQ:
http://www.netaxs.com/home/v/nfilter/faq.html