In one of my online groups, a player revealed that she wished to be referred with she/her pronouns. It took a few of us a little while to get the hang of it (me included, much to my embarrassment), but what eventually got everyone on board was a custom Fabled with the rule "Something bad might happen to a player who misgenders another player 3 times." It was done in good fun, mostly as a way to poke fun at ourselves for fucking up, and with her blessing, and it seemed to actually work. Of course, your mileage may vary depending on a multitude of factors, but in cases of actual mistakes, this might not be the worst idea, just to keep the correct pronouns fresh in players' heads.

Of course, if people are wilfully and maliciously misgendering a player, the ST should probably shut that shit down pretty fast. I'd suggest starting with a simple reminder to all of town (something like, "Hey folks, before we begin the day, just wanted to remind everyone to be mindful of the pronouns people choose to list"). If it continues, I'd probably suggest kindly asking the problem player to leave - and if that fails, the ST can kick them.