Will it start without the lanyard on the post, or it just keeps running after you have removed it? It’s strange either way, but I’d be more concerned if it will start without it, as that would lead me to believe you have an aftermarket MPEM installed as well. The factory computers will not engage the starter at all without the proper key attached, but the aftermarket ones will work with any key on the post.
If it just won’t shut off with the start/stop button, I would replace the switch. It’s not a terribly difficult process, and I’m sure Westside Powersports has a good used complete assembly laying around.