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I'm not sure that you can block HTML mail, but through the preferences you are able to stop html messages from displaying pictures.
 
Not entirely sure if you can block html mail completely, but there's a couple of things you can do:

The easiest is to block remote images in html mail. That's done in Mail preferences -> Viewing.

You can also force Mail to display all html mails (with a plain text alternative) as plain text (same as the View menu -> Message -> Plain Text Alternative) by typing this command into Terminal (whenever Mail isn't running): defaults write com.apple.mail PreferPlainText -bool TRUE
 
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