Procmail (a mail processing language) is very useful for stuff like this. It’s a bit difficult to learn, but there’s a lot of information online about it.
Unfortunately, my kbase docs on it are currently a bit out of date, at least as far as dh-specific stuff. I’m going to be fixing that Real Soon (my much-needed and too-short vacation is over tomorrow, so I’ll get cracking soon).
Other than that, reporting the mail to the actual origin of the message is a good idea; using spamcop might be helpful in determining it, but i wouldn’t use spamcop to report it, as it may not actually be “spam”, and you might accidentally send a complaint to somewhere unrelated.
If you send in the header section of the message to support, they could help you out with figuring out the origin as well.
Are you sure it’s a flood attack and not a virus of some sort? Is there a binary attachment that comes with the email? Is the ‘From:’ header always the same? What about the Return-Path: header?