Two solutions:
a) set up a filter do delete any messages with:
Content-Type: text/html;
in the header or body.
Drawback: it would delete even messages like this one.
b) set up two filters, one to delete messages with:
Content-Type: text/html;
in the header, and one to delete messages with:
Content-Type: mulitpart/alternative;
in the header.
Drawback: this might delete even some non-html messages