You need to set the stage before the adventure starts. Let the party know what kind of adventurers you are looking for and what kinds of motivations will fit with what you plan to do. This doesn't need to include alignment restrictions, but I think it's a good idea.

Party is a CE dread necro and a paladin is pretty extreme on the non-functional end (and will be stopped if you put basic alignment restrictions in place), but "I was planning on running a morally ambiguous campaign of cutthroat opposition to Iuz (and minor larceny/banditry) in the bandit kingdoms and player A wants to run a Dudley Do Rightesque goody two shoes lawful stupid cavalier is also going to cause trouble or a devil-may-care greedy rogue in Pendragon-esque campaign. Let players know the ground rules first and they can come up with a concept that fits the setting and campaign. Otherwise, they'll find out "the in character thing for me to do here is say, 'a pox on you all' and walk away."