I have had a number of different procmail recipes in place to block attachments.
None of them have blocked all undesirable attachments, but this one seems to
handle most of them.  Note that I block all of the useless attachment types, plus
.exe and .mp3.  Most mp3 files are too big to be delivered anyway - this saves
them from wasting bandwidth.  Just copy this into your /etc/procmailrc file and
it will immediately re-route the message and attachment to the bit bucket.

Unfortunately, if the mime encoding isn't correct, the attachment can still get through.

LOGFILE=/var/log/procmail
:0B:
* ^*filename.*\.scr
/dev/null
:0B:
* ^*filename.*\.vbs
/dev/null
:0B:
* ^*filename.*\.bat
/dev/null
:0B:
* ^*filename.*\.com
/dev/null
:0B:
* ^*filename.*\.exe
/dev/null
:0B:
* ^*filename.*\.pif
/dev/null
:0B:
* ^*filename.*\.cmd
/dev/null
:0B:
* ^*filename.*\.mp3
/dev/null