I wouldn't call that undermining freedom of association. You don't have to follow and can block people on twitter, for example. Just because a nazi is there doesn't mean you also being there means you support nazis by association, that's nonsensical.

That said, section 230 should not be overturned, and doing so opens a whole host of problems. Companies not associated with the government should have the ability to do what they want with their platform, within the boundaries of law. The effects of social media and its dominance in information, discourse and it's algorithmic attempt to keep you in a bubble is likely not good for society, and will have consequences we still can't foresee.