I was sitting here thinking about your problem. and actually if the solenoid plunger is stuck in it would be turning over even with the key off. just like you can push a slam button in with the key off and it will engage the starter. anytime the plunger is pushed in till it makes contact the starter will engage. even with the key off. It could be a low voltage problem. just like the old chevy corvettes , if the battery was low it wouldn't suck the plunger back out and they would just keep turning over till you disconnected the battery. Actually the button wire is the only thing on the starter that has no power when the key is off. I would make sure it has a good charge showing 12 to 13 volts in the battery, if it does,and its not the starter button or button wire,,, then my best guess would be the jack shaft gear is stuck out on the ring gear and thats what is holding the solenoid plunger in so its making contact.. i would first disconnect the starter button wire on the starter. the lil blade plug in above the solenoid. then turn the key on and see if it still does it. if it does then, you know its not the button. then i would pull the solenoid cap and then the primary cover or jack shaft cover if its belt drive.. if anyone has messed with the handle bar controls it could be the button wire is shorted.. bout all you can do is start an elimination process by checking everything in order.