That's why you just say, "good thanks," and NEVER EVER ask them in return.

Many people ask how someone's day is because they actually want to tell you about their day. And the normal thing is for people to return the question. So you get trapped into listening to someone complain about their day because you retuned their small talk.

Instead, just don't engage in the small talk by answering but not asking in return.

A lot of people get pretty put off by it, but they're either not able to articulate why, or they realize that "but you didn't ask me about my day!" is a stupid thing to be mad about. So the end result is those people find a different victim who's attention they can consume, and I am spared.