nearly all competitive twitch skill multiplayer games (counter strike, TF2, day of defeat, battlefield) institute ping limits of 100-200 at most. Granted these games have a much large following compared to cRPG so I dont know if cRPG can get away with limiting where people want to play.
In most instances I've seen, the high pingers are just leeching and not providing any benefit to their team. 300-350 would be a reasonable limit for cRPG