ColinChapman
Longtime RPG Freelancer/Designer
I'm with Mistwell. I pitch a given campaign to my players first. They then create character concepts appropriate to said campaign and we discuss how it'll all work. They don't come to the table with preconceptions as to what they are going to play before that point because it would be a colossal waste of time. They trust me to run fun, interesting games. I trust them not to create campaign-inappropriate characters.
Not that it has ever happened in my experience, but if someone wanted to play something that really didn't fit the campaign, I'd suggest alternatives as close conceptually as I could (if possible) that did suit, but I'd still have no compunction in saying, "Sorry, that concept just doesn't fit." if they continued. If they still continued, they'd be shown the door. Being so wedded to a single character concept that you're prepared to ignore the setting and potentially ruin the feeling and theme for the DM and the other players means you're not the sort of person I'd want to play with. Fortunately, that has never been an issue in my groups.
Not that it has ever happened in my experience, but if someone wanted to play something that really didn't fit the campaign, I'd suggest alternatives as close conceptually as I could (if possible) that did suit, but I'd still have no compunction in saying, "Sorry, that concept just doesn't fit." if they continued. If they still continued, they'd be shown the door. Being so wedded to a single character concept that you're prepared to ignore the setting and potentially ruin the feeling and theme for the DM and the other players means you're not the sort of person I'd want to play with. Fortunately, that has never been an issue in my groups.