By law. A simple law like those that govern utilities like phone lines, mail and Internet access. These exist in most western countries. Your phone conversations, letters and self-hosted personal website cannot be censored by anyone without a court order.

Simple to add to Section 230 a line that reads

"For the purposes of Section 230 immunity all platforms shall moderate content in a manner that conforms to the 1st amendment of the US Constitution as if they were a government agency. Any platform that chooses to suppress, moderate or take down 1st amendment protected speech shall be considered publishers of all user generated content and loose the liability protections granted by this section"