The same way we regulate any business and mandate they abide by regulation. Or the same way we force phone companies to be regulated as common carriers. The same way your power utility cannot cut you off based on your political opinion. All it takes is political will.
I'm not entirely sure that's the best model for today, but that's the historic model for a private business that's compelled to allow all legal speech. That's why UPS and FedEx still have to deliver to the New Order (the current incarnation of the American Nazi party).