sgtjesse wants to know how to approach difficult players, and I'm gonna guess that he means about their difficult behavior, not how to invite them to games or ask them out or something.
There's a lot of discussion lately about whether or not it's ok to kick people out of a game or not. The one thing that almost everyone agrees upon is if you'd kick someone out of your house for something, and I'm talking about social behaviors here, not in game behaviors, then it's perfectly valid to kick them out of your game.
Where this gets a little fiddly is what constitutes a social behavior and what's an in-game behavior. Is constantly bringing up material that makes other players uncomfortable an in game or out of game behavior? So, let's start there with some simple social rules to govern in game behavior.
1. Discuss appropriate subject matter.
The game theory crowd refers to this as the Lines and Veils discussion. Where are the lines when it comes to subject matter? It's best to know beforehand. If something really bothers someone and they'd rather not have it come up in the game fiction then it's best it comes up early, so that there's no surprises later.
A veil is something that is ok to come up, but players would rather it fades to black instead of being described in detail. Sex often comes up as a veil, players might be ok with characters having sex, but don't really care to hear it described in loving graphic detail.
2. Discuss the kind of game you all want to play and decide how much focus will be given to the styles and themes.
I've harped on this before, but if everyone is interested in a game where party unity is key and one player wants to play the thief who steals everyone's shit, there's probably gonna be a problem. If that player knows at the outset that stealing from the party isn't ok, then they have no room later for the "but that's what my guy would do" argument. Putting this to the forefront during character creation let's that player know that they should make a character that wouldn't do that.
My lunch grows ever shorter and I find myself repeating a lot of stuff, so I'll boil it down to one magic rule.
Discuss what's ok and what's not ok before the game even starts.
I'll follow this up with a post later about how to talk to problem players about their behavior when these rules break down later.