Blocking GIF Spam

[Kay Sommers <ksommers () VCU EDU>]

Has anybody had success in blocking the market advice spam messages that
have been appearing lately?    The problem is that the messages are
images, and while our Brightmail scanner has the ability to devise a
signature for attachments, each of these images is just a bit different
which causes problems for signature matching.  The senders are usually
different too.  There must be thousands of bots invoved.
If we block the GIF files, the end users will still get an empty message
which might actually be worse (more confusing for many of the users).
Any ideas for a good solution?