Jason, I would be interested to hear if there is any malice involved. Do you have internal recipients trying to actually spam, or are they simply forwarding mails (possibly released from quarantine) to their home addresses or close friends? The fact that you are releasing them would rather suggest the latter.

If so, remember that you can make responses to your internal community that you can't to the external community. Automated responses that we would consider backscatter could be quite helpful to your internal recipients. At the very least, if Cisco can't adjust CASE to your liking then you might be able to brush the problem under the carpet with an automated "this e-mail breaks {organisation} policy - please re-draft".

I have a similar problem in the opposite direction with spam forwarded in from home addresses, but the mails simply sit in a quarantine until they are flushed. The recipients don't come looking for them, and I currently have a zero percent complaint rate on that score.