Alternative payment processors, counter petitioning and counter social media campaigning to pressure payment processors to not listen to them, or even legal action if it comes to that. There's already related ACLU petitions for similar discrimination https://action.aclu.org/petition/mastercard-sex-work-work-end-your-unjust-policy (Good to sign if you're a US citizen, and can reasonably cover this too, 74k out of 100k signatures needed so far), and now also a change.org petition targeting visa and mastercard for exactly this, https://www.change.org/p/tell-mastercard-visa-activist-groups-stop-controlling-w... . The petition they responded to didn't have that many signatures even, its entirely possible to garner more to try and counter them.
Pressuring payment processors in any way possible to ignore them is important, to make sure the loonies aren't the only voice that gets a say. If nothing else it shows that's not the only pressure effecting their business. Never mind the question of whether these things could classify for an antitrust lawsuit, or that there's some level of investigation reportedly happening in the Japanese government
Moreover, if there was ever a time to take a stand, its now when everyone is already pissed off about the steam stuff. Makes it easier to rally them