Me:  "The game setting, or this region anyway, is 99% Human, <1% Dwarf.  You can play a Gnome or a Halfling, but no one has ever seen one and it will be a bit hard.  Every other race is a hated enemy.  Basically everyone is going to be Human or Dwarf."
Prospective Player:  "Can I play a half-troll?"
Me:  "No.  Human or Dwarf."
PP:  "Why not?  I like playing odd races!"
Me: "You will be seen as a monster by everyone and killed on sight.  Play a Human or a Dwarf."
PP:  "How about a Half-Ogre?"
Me:  "No.  (repeat explanation)"
PP:  "Can I play a Gnoll?"
Me:  (frustrated, explain setting again)
PP:  "Can I play a (equally unacceptable monster race)?"
Me:  "Maybe you shouldn't play in my game."
PP:  whine whine whine, "you're not being fair!"
Me:  (happy not to know this person anymore)
You know, if he wasn't going to be happy playing a Human or a Dwarf, then he wasn't going to be happy playing in the game.  At some point he should have gotten the clue and decided that perhaps he shouldn't play that particular game.  Or if he really wanted to play with that particular group of people, then he should have adjusted his expectations and made an effort to play the setting.  I'm not interested in coddling or negotiating with people who can't get that.
"Oh sure.  You can play a half-troll.  And despite the xenophobic nature of the local culture, YOU will be welcome in every town and inn, trusted by everyone and even loved by small children and animals.  Just because you're a PC."
Yeah, sure.