What is “harmful behavior” when it comes to words? You may have your own definition, but I guarantee that you can’t get a supermajority to agree with it.
(There are some basic things like fraud and such that are already illegal.)
Most people don’t actually want to ban “harmful” speech except for the speech of their political enemies.
Nobody mentioned speech. Infinite scroll isn't speech. Engagement-maximizing algorithms that push increasingly extreme content to keep you on the app longer ain't speech. The business model that makes outrage more profitable than accuracy isn't speech. You can have complete freedom of expression and still regulate that a platform isn't designed to psychologically exploit the people using it. These are not the same thing