While I've never had to deal with this type of player before, I'm hesitant to agree that it poses as big a problem as the much more common hyper-destructive player. Yes, taking the moral high ground on every issue may bring the boy/girl scout into conflict with the other players, but it also depends on his/her attitude. Tom plays a lawful good character in Ross' New World Campaign while the other characters prefer to play it morally gray, but he's not exactly argumentative about it when the others try to do something questionable. If a player's behavior is disrupting the game--if he's belligerent or rude to the other players because of how their characters choose to act--then he's probably not someone you want to be playing with in the first place. However, if it's just an in-game conflict between characters that's holding things up, I would do one or both of the following: make the decisions that the character has to make harder--i.e., don't give him a "good" choice--or emphasize the consequences of making the "good" decision. Personally, I think that in an interesting story, the moral high road is often difficult to take, or difficult to distinguish. If you're having problems with a character who is too "good," put him in a situation where the best he can hope for is to take the least questionable action. Make it hard to be good.
