Quick even MS allows you to insult a players play.
To a certain degree.
They are more liberal with usage of the rules, but thats because its a bigger crowd and not everyone gets it that the other person doesn't really mean it.
I mean if I have a back and forth with most players here they will get it that I don't really mean it, some won't but a most get it. If you feel like someone is going too far you can always just tell them "Hey stop" unless they are huge asshole I think most will respect that on some level and back down at least a bit.
- - - Updated - - -
Mafia is about communication and when you start banning certain ways people communicate you limit the ways in which people play.
I agree with Crypt on the whole point about we aren't robots.
Does that mean we allow all forms of people Ad Hom? No. It depends on intention of the player.
Is the player doing so to actually legitimately hurt you? That isn't okay.
Is the player acting within their role/win con to further it? Yes, than usually okay.