While I tend to agree that it would be great to be able to block all
HTML you can do this today.  LISTSERV(tm) has the capability to filter
messages any way you want with the "POST_FILTER" exit.  We used this for
some time (using the unsupported sample POSTFILTER filter) to block all
multipart mail messages as well as common uuencoded bodies.  I'm sure
this code could easily block any HTML types, allow only plain/text,
whatever.
   This feature is documented in the Developer's Guide.

   Marty