This question is only really relevant to Gnus 5.8.x or greater, as
   Gnus did not have native MIME capability before those versions.
   
   The variable mm-discouraged-alternatives may be helpful. If you do the
   following:
(setq mm-discouraged-alternatives
      '("text/html"
        "text/richtext"))

   It will always show the text/plain part of a multipart which has both
   a text/plain and a text/html part, for instance. However, if there is
   no plaintext part, Gnus will still process the HTML. If you wish to
   prevent that, you can do one or the other of the following:
    1. Use mm-inline-override-types.
       If you set this variable to the same value that
       mm-discouraged-alternatives is set to in the above example, the
       HTML will initially be buttonized instead of displayed.
    2. Use mm-inline-media-tests.
       Dale Hagglund <rdh@best.com>writes:
       I added the following to my .gnus file not too long ago.
(setq mm-inline-media-tests
      (cons '("text/html" nil (lambda (h) nil))
            mm-inline-media-tests))
       [...] text/html parts are not automatically displayed using w3 or
       any other browser; they are given buttons, and the html text is
       shown inline. if you click the button twice, gnus starts any
       text/html viewer specified in you .mailcap file.
       
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