[Mailman-Users] Filter replies that contain the digest

Hello, I've searched around the web for this answer and haven't found anything. My problem is this:

I have a list that sends out a digest monthly, so if one of my users reply to that digest and include the entire digest in their reply the list is blasted again with a large "digest size" email. When other's reply to their email, it continues to grow (I already filter on size, but I'd rather be more specific to just emails that contain the digest). I'd like to prevent users from replying with the entire digest in the body of their email. My initial impulse was to add a header to the digest and filter on that header, but I wonder if there's a better way to control this behavior using mailman.

Any ideas/comments are welcome.

Thanks,

CE