Player uses a slur to describe a card
Depending on the player, often times responding with a simple “Hey, not cool” can be all you have to say to change behavior, if you're in a situation where the community hasn't developed a position like the one William described. You don't have to go fire and brimstone to effect change, and sometimes if you don't need to people will respond more positively.
I've handled several situations at Regular REL where somebody will let whatever perjorative slip, and when I responded with a sharp look and “Hey, not cool” I've actually had players come up to me later, apologize, and then be more aware of their own behavior later on. I even saw one individual I spoke with like that mimic my actions, and help correct the behavior of another player.