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