The problem isn't HTML, it is its users. People who can't quote properly, people who end every single sentence with an emoji, etc. In a nutshell, people who have never experienced Usenet never learned how to communicate properly with a keyboard.
The fix for this is to configure your email client to auto-answer a rejection message when it receives anything other than plain text. You can always blame your antivirus for that.
Help people with disabling HTML email. With a little bit of luck, they won't remember how to enable it again. And hopefully, since they won't be distracted any more with fancy formatting, animated smiley and which GIF-meme-to-use-for-this, hopefully they'll focus on words and reinvent ascii art.