If it has electronic ignition, there is only one wire from the unit, if this wire is earthed out it should cut the ignition, it is normally a black wire connected to the back of the unit, if it does not stop check the wire is not broken internaly, make sure when you earth it out it is earthed to the engine block,this should rule out the ignition switch, does it now stop? if not you will need a new ignition unit as there must be an internal break.
Husqvarna man, January 2010