In this case, the market. This isn't a town square or any other public space. It's private enterprise. If Spotify loses enough revenue from it, they'll probably do a song and dance about ethics and duty to blah blah blah, to justify evicting him. Right now, the math is on the side of Rogan. Maybe it changes. Maybe it doesn't. My money goes elsewhere, when I find content to be too inflammatory.
Can you give a non-hypothetical example of something online that is a public space?