See, here it makes it sound like you’re giving players a choice. “Here is a thing I want to do, cool?”
What you described was setting. This is what players should be agreeing to before they finish their first character. Their interpretation is going to change things, yes, but I cannot, e.g. go to Arx and decide my character is half-elven.
Nor can I go to an Elf-Only Inn RP channel as Spock. This is not a lack of “buy-in”, this is outright disrespect to the game. (Or hilarious.)
There are only a few things you can do, and most of them are corral the players until they agree to play the game before them. The other is throw them off or close the game.
You can also luck out and not have this be a game-closing problem. A solid player base will help you overcome many obstacles.