I like to present my players with two things at the start of a campaign. A list of rules for character creation
A short pitch of the campaign, describing themes and opposition because you can't ban everything.
"That aquatic druid might sound really interesting but this campaign is gonna be about a zombie invasion it might not be the best place for that character to shine" is the kind of thing I will say. Or "we are going for an Arabian Nights feel, maybe your Samurai isn't a good fit, but here's how your ideas might fit in with the theme." My goal here is to start a discussion, maybe the samurai will end up in the game, but the discussion need to happen.
If a player insists and start trying to badger me, that is the point where the creation rules are handy, being explicit on why you don't allow some things is a good thing here. If he continues and start flipping tables, no game is better than a bad game.