Mike, a question.
Can't your abuse department work directly with Comcast NOC to resolve this? When I worked for MSN Broadband that was normal business practice, at least with Qwest and AT&T Broadband. Of course that was before Comcast moved into Utah. But I would think that in the spirit of providing good customer service, even Comcast would work with large service providers at a NOC level, to correct spamming issues, etc.
The last thing I want to do is shoot anyone, but it's on the list...