No, really.
That word is everywhere in Mafia, and it's pretty ridiculous. I encounter it far more than anywhere else on the Internet other than White Supremacist web forums, and far more than Starcraft 2 ladder games. The word is widespread enough that it's become a meme of sorts in Mafia.
I suspect the inability to right-click report players directly through chat as you can in ordinary games is largely responsible. I don't know that it's within the technical capability to reinstate right-click reports without revealing Battle.Net names but it seems pretty clear that the current situation is unacceptable and the current mechanism of expecting people to wait until the end of the game, look up the real Battle.Net name, match it with the player yelling "NIGGER" and then file a report in the scorescreen.
I see no reason why player reports of players using disparately offensive language should go unpunished by the Mafia mods as well. Other prominently offensive comments I've encountered are calling everybody fag/faggot, derogatory comments about Jews (usually involving lethal gas), and occasionally graphic descriptions of sexual conduct (though personally I find the last one far less concerning).
Maybe just reading my post will be enough to get people to stop doing it. But honestly I'm getting pretty fed up by now, and I'd love to find a way to make Mafia a more positive experience.