Count yourself lucky your clan didnt actually lose any points because its a playoff, just a forfeiture of the match.
There is no point penalising just the person involved, as you have demonstrated so well if a player wants to play so desperately then they can find ways around the basic login system, if the clan involved isnt penalised then they have nothing to lose by risking the merc/banned/whatever player from playing an important match and may continue to take the chance even when caught. If a clan is caught using unauthorised accounts then they should deserve to be penalised as well (yes this bit is in the rules as well, maybe you should take a look and familiarise yourself with them), if a clan isnt willing to use these unauth'd players then there would be no mercs/etc.